More than 300 cryptocurrency traders reported over ¥100 million in miscellaneous income in their 2017 tax returns as a result of trading bitcoin and other cryptocurrencies, according to data released last week by the National Tax Agency.

While 331 of them each reported miscellaneous income of over ¥100 million from cryptocurrency trading, netting the government billions of yen in new tax revenue, there is still widespread belief that many investors have yet to report their cryptocurrency investment earnings to authorities.

"Many people made profits on cryptocurrencies in 2017, but many reinvested and lost money in the market downturn," said Yasuhiko Sugiyama, a tax accountant knowledgeable about cryptocurrency taxation. "I think there are many people who may be unable to pay these taxes."

Confusion over exactly how to calculate earnings and report taxes obfuscates the true number of extremely wealthy investors in Japan, Sugiyama added.

Tax authorities issued new rules at the end of 2016 stating that earnings from cryptocurrencies are to be treated as miscellaneous income, the classification that includes royalties and paid speeches.

The addition of cryptocurrency income has bumped up tax revenue from the miscellaneous category by about ¥30 billion, a 50 percent increase from the year before.

The top taxation rate for cryptocurrency earnings is 55 percent, but depending on the circumstances it can be as low as 5 percent.

While the new source of revenue is a welcome development for a government struggling to balance its books, authorities are still on the hunt for tax avoidance.

"We think there are more people who made income over ¥100 million," said an official from the National Tax Agency. "We are now in the process of finding out whether people are hiding profits from cryptocurrency."