If you trade crypto assets (also known as virtual currencies) frequently, your annual transaction count can easily run into the dozens or even hundreds. As a result, calculating gains and losses becomes complicated, and unintentional underreporting can happen.
This raises an obvious question: is it safe to leave unreported crypto income unaddressed, or will the tax office eventually catch it?
Today, the data available to Japan's tax authorities is more extensive than ever, including information provided by exchanges (order-matching marketplaces) and transaction records held by financial institutions. This holds true even for investors using overseas exchanges — using a foreign platform does not make transactions or fund movements completely invisible to the authorities.
Note on the current tax framework (as of July 2026): Under Japan's tax law today, gains from selling crypto assets are treated as miscellaneous income and subject to aggregate (comprehensive) taxation rather than separate self-assessment taxation. On July 15, 2026, an amended Financial Instruments and Exchange Act (FIEA) passed the House of Councillors, which will move crypto assets under FIEA as a financial product; a flat 20% separate self-assessment tax rate is expected to apply starting the fiscal year after the amendment takes effect (projected January 2028). Separately, losses from crypto trading can currently only be offset against other miscellaneous income — they cannot be offset against employment income or other income categories.
There have been real-world cases of this going badly wrong. In one instance, a man in his 40s living in Tokyo with his wife and child failed to report profits from crypto assets and ended up facing more than ¥200 million (approx. USD 1.33 million, at roughly ¥150/USD) in back taxes and penalties.
He didn't think he needed to report the portion he had converted between crypto assets, so he only filed a return for the amount he had cashed out. Last September, the tax office pointed out the underreporting, and the total additional tax — including the penalty for underreporting — came to more than ¥200 million.
This article explains the mechanisms and background behind how unreported crypto income gets discovered in Japan, along with key points to keep in mind when filing.
Common ways unreported crypto income gets caught
In principle, unreported crypto income is designed to be discoverable. The reason is that the tax office now has access to a growing pool of information, making it easier to spot inconsistencies between what's reported and what actually happened.
There are several routes through which underreporting comes to light, including the following:
- Discovered through exchange data
- Discovered through linkage with a taxpayer's My Number (national ID)
- Discovered through bank account deposits and withdrawals
- Discovered through social media posts
- Discovered incidentally during a broader tax audit
The list of ways underreporting can surface is essentially endless. Below, we focus on the three mechanisms most relevant to the average trader.
How exchanges share data with the tax office
Japan's domestic crypto exchanges are required to provide user transaction data in response to inquiries from tax authorities. This means transaction amounts and trading histories can be checked, and any inconsistency with what was reported may be identified.
Domestic exchanges also require identity verification when opening an account, and in most cases the submission of a My Number (Japan's national identification number) as well. Because of this, transaction data is managed in a way that is tied to a specific individual — it isn't something that can be used anonymously or with complete freedom. If there's a gap between what's reported and the actual trading history, it can surface during a tax audit or similar process.
Furthermore — as covered in more detail below — Japan has tax treaties and information exchange agreements with a large number of countries, and frameworks exist for receiving information from overseas financial institutions under certain conditions. This means that even for investors using an overseas exchange, transactions are not necessarily beyond the tax office's reach.
How bank deposits and withdrawals reveal profits
When crypto assets are sold and converted into Japanese yen, that money is, in most cases, deposited into a bank account. The tax office can request transaction statements from banks when necessary, so a pattern of large deposits or unusual account activity may prompt a closer look.
Banks are obligated to disclose deposit and withdrawal histories when the tax office makes a formal request. By reviewing which accounts received how much money and when, it becomes possible to infer whether gains from selling crypto assets occurred.
In years with heavy crypto trading activity, large deposits and withdrawals tend to occur even when no net gain was realized. Because this pattern differs from typical salary income or other ordinary income flows, it can be a trigger for the tax office to decide a closer check is warranted.
When bank account data is cross-referenced against exchange data, discrepancies between what was reported and what actually happened become apparent. For this reason, bank account activity is one of the most common routes through which underreporting is discovered.
How account information is shared even from overseas exchanges
Using an overseas exchange does not exempt an investor from their filing obligations.
Under Japanese tax law, residents are, in principle, taxed on their worldwide income. This means that income from crypto asset transactions — whether earned domestically or abroad — is subject to reporting.
Individual category
Definition
Scope of taxable income
Resident
Resident other than a non-permanent resident
An individual who is not a non-permanent resident and meets either of the following:
• has a domicile in Japan
• has continuously had a residence in Japan for one year or moreAll income earned both within and outside Japan
Non-permanent resident
A resident who meets both of the following:
• does not hold Japanese nationality
• has had a domicile or residence in Japan for a total of five years or less within the past 10 yearsIncome other than foreign-source income, plus any foreign-source income that is paid within Japan or remitted from abroad
Non-resident
An individual who is not a resident
Japan-source income only
Source: Who Is Liable to Pay Tax as an Individual | National Tax Agency (in Japanese)
Today, countries have built out systems for sharing tax-related information with one another, and frameworks such as the Common Reporting Standard (CRS) for the exchange of financial account information are already in operation.
In addition, when funds are transferred from an overseas exchange into a Japanese bank account, that deposit and withdrawal history is recorded by the domestic financial institution. This information may be confirmed in the course of a tax audit, and any inconsistency with what was reported can be identified.
In short, simply using an overseas exchange does not reduce a taxpayer's filing obligations in any way.
Audit numbers and back-tax assessments for unreported crypto income
Data published by Japan's National Tax Agency (NTA) shows that crypto asset transactions have become a focus area for tax audits.
According to figures released by the NTA for fiscal year Reiwa 4 (roughly April 2022 to March 2023), 615 on-site audits were conducted on individuals engaged in crypto asset trading, and underreporting was flagged in 548 of those cases.
<Status of audits of individuals engaged in crypto asset transactions>
➤ In fiscal year Reiwa 4, 615 on-site audits (special and general) were conducted, up from 444 in the prior fiscal year.
➤ The average amount of underreported income per case was ¥30.77 million (approx. USD 205,000), down from ¥36.59 million (approx. USD 244,000) the previous year. The total amount of underreported income across all cases reached ¥18.9 billion (approx. USD 126 million), up from ¥16.2 billion (approx. USD 108 million).
➤ The average additional tax assessed per case was ¥10.36 million (approx. USD 69,000), down from ¥11.94 million (approx. USD 79,600). The total additional tax assessed came to ¥6.4 billion (approx. USD 42.7 million), up from ¥5.3 billion (approx. USD 35.3 million).
The number of audits increased compared to the previous fiscal year, suggesting that crypto assets remain a focus area for the tax authorities. The scale of the flagged cases and the associated back-tax assessments also indicate that crypto-related audits are being carried out on a meaningful scale.
This is likely due to improvements in the data infrastructure available to tax authorities — including information provided by exchanges, inquiries into bank account activity, and international information-exchange frameworks. As a result, it has become easier than before to verify whether reported figures match actual trading activity.
Regardless of the amount involved, any inconsistency in a tax return may be identified in the course of an audit.
Penalties for failing to address unreported crypto income
If profits from crypto assets go unreported and the tax office later identifies the issue, the taxpayer becomes liable not only for the original tax owed but also for late-payment tax and additional penalty taxes.
Late-payment tax (entaizei) is charged on tax amounts that were not paid by the statutory deadline. It accrues based on the number of days the payment is overdue, so the longer the issue goes unaddressed, the larger the burden becomes.
Additional penalty tax (kasanzei), on the other hand, is imposed when there is a problem with the return itself, and comes in several forms, including the penalty for failure to file, the penalty for underreporting, and the heavy additional tax for cases involving concealment or falsification. The applicable rate depends on the nature of the underreporting, and a higher rate applies if the tax office determines the omission was intentional.
These amounts are added on top of the original tax owed, meaning the total payment due can grow substantially. In fact, published NTA audit results show that crypto-related underreporting cases have resulted in significant additional tax burdens once late-payment tax and penalty taxes are factored in alongside the original tax.
That said, a system exists to reduce the penalty tax rate for taxpayers who voluntarily file an amended return before the tax office points out the issue. In other words, it pays not to leave a known error unaddressed — filing a proper amended return promptly can keep the burden within the scope set by the reduced-rate system.
What to do if you discover unreported crypto income
Underreporting can happen to anyone, and taking action as soon as you notice it can help you avoid unnecessary penalties.
What matters most is that, once you realize crypto income has gone unreported, you organize the facts and move quickly to file the appropriate paperwork. There are broadly three steps involved.
Review your crypto transaction history to calculate your profits
The first step is to organize your transaction history into objective data.
Most crypto exchanges let you download an annual transaction report or account statement. Start by pulling the data for the relevant tax year and compiling it into a spreadsheet. From there, identify every transaction that could be subject to tax — not just outright sales, but also crypto-to-crypto swaps and payments made using crypto assets.
Taxable crypto income isn't limited to simple buy-and-sell transactions. For example, the following also count:
- Swapping one crypto asset for another (e.g., BTC → ETH)
- Using crypto assets to pay for goods or services
All of these need to be factored in when recalculating how much profit or loss occurred, and in which year.
File voluntarily with the tax office
If reviewing your transactions reveals that income went unreported, you'll need to file either an amended return or a late return.
Filing voluntarily, before the tax office points out the issue, comes with a reduced rate for the penalty for failure to file.
If a late return is filed voluntarily before receiving advance notice of a tax audit, the penalty for failure to file is calculated at 5% of the tax owed, in addition to the tax itself.
Source: When You Forget to File Your Final Tax Return | National Tax Agency (in Japanese)
If a late return is filed after receiving advance notice of a tax audit, but before the taxpayer could reasonably anticipate that the audit would result in a formal determination, the penalty for failure to file is calculated at 10% of the tax owed, in addition to the tax itself.
Source: When You Forget to File Your Final Tax Return | National Tax Agency (in Japanese)
Because of this, it's important not to leave the matter unaddressed once you notice an issue — instead, take the necessary steps promptly at your local tax office.
Consult a tax accountant
If you have a large number of transactions, or need to go back and recalculate several years' worth of activity, consulting a tax accountant (zeirishi) who specializes in crypto asset taxation is worth considering.
Calculating crypto gains and losses involves a number of judgment calls — how acquisition cost is determined, how transfers between exchanges are treated, how staking rewards are handled, and more. Working through it on your own raises the risk of calculation errors and further underreporting.
For that reason, it's generally the safer path to consult a specialist with crypto expertise before filing.
In summary: act quickly once you notice unreported income
Under Japan's system, unreported crypto income isn't something you can simply leave as-is. It's important to organize your transaction history, recalculate your income, and file an amended or late return as needed.
Voluntary action may also qualify for reduced penalty rates in some cases. Accurately assessing your situation and taking the proper steps is, in the end, the best way to keep your financial exposure from growing.
If your transaction volume is high, or your gain/loss calculations are complex, consulting a tax accountant who handles crypto assets can also be a worthwhile option.
Understanding the tax rules is central to handling crypto assets responsibly. Organizing your transactions and addressing any issues early is the key takeaway here.
This article is for informational purposes only and does not constitute financial or investment advice. Please consult a qualified professional before making investment decisions.
