Holders of stock options failed to report 51.78 billion yen in income earned by exercising those options over a two-year period through June, according to figures released Tuesday by the National Tax Administration.

The tax office levied 18.47 billion yen in penalty taxes after conducting its first survey on stock options, which are incentives given to company employees.

Tax authorities regard any gains made after exercising the options as a "temporary bonus" and levy tax against those gains.

The tax authority looked into 1,815 declarations for stock options and found underreporting in all but 13 cases. The average amount of underreporting was 28.53 million yen and the average penalty came to 10.18 million yen.

The survey also notes that taxpayers have filed 11 lawsuits challenging the policy of taxing stock option gains as regular income, arguing they should be taxed as one-time income, which carries a lower tax rate.

In a separate study, the tax administration found self-employed workers underreported their income by a total of 1.183 trillion yen in the year through June, 222.4 billion yen less than the corresponding figure last year.

Tax-related penalties, including fines for tax fraud, totaled 167.5 billion yen, 4.13 billion yen less than the previous year.

The survey looked into 831,000 tax declarations and found 629,000 cases of underreporting, the National Tax Administration said.

In terms of industries, pachinko parlor operators were the worst offenders for the first time in four years, with the average amount of underreporting per case totaling 68.99 million yen.

Builders were next, averaging 29.25 million yen per case, followed by moneylenders at 28.7 million yen, down from the No. 1 position last year.