Where to take your valuations from

Crypto has no single official price the way a listed share does. At the same instant, different venues with different order books quote different numbers. For events where yen is not directly involved — a swap, or receiving a reward — you have to derive the yen value yourself, and that raises the question of whose price you take.

The practical principle is consistency. Using whichever venue's price happens to be favourable for a given transaction is not a position you can defend. Decide on an approach — the venue you mainly use, or the venue where the transaction actually happened — and apply it across the year. Then write that policy down.

You also need a convention for the moment. Using the price at the instant of execution and using a set time of day produce different numbers. Japanese exchanges' annual statements usually carry yen values already, and for transactions inside that exchange those are the natural figures. The difficulty is with swaps that never touch yen and with events outside an exchange entirely.

Thinly traded assets deserve particular care. Where an asset barely trades, the quoted figure may not reflect a price at which you could actually transact, and different data sites aggregate different venues, so their quotes can diverge sharply. How to value such assets is a genuine judgement call.

Keep a record of the source you used and the figures you took from it. Years later you may need to explain why a yen value is what it is, and data sites change their displays or stop serving historical data. Saving the page or the downloaded data at the time solves that. On which source is appropriate, read the National Tax Agency's guidance and take remaining doubts to a tax accountant.