About 300 post offices in Japan have failed to register part-time employees for the social insurance system, leading to a premium shortfall of about 500 million yen, the Board of Audit said Tuesday.

Although the board looks at discrepancies in social insurance premiums every year, this marks the first time it has delved into a working situation involving part-time postal employees.

The roughly 20,000 post offices nationwide employ about 80,000 part-time staff a day, and the Board of Audit called on the Social Insurance Agency to beef up its checks of whether premiums are properly collected.

Under the Health Insurance Law, workers are required to enroll in the social insurance system. The premiums are supposed to be jointly paid to the local social insurance office by the employer and the employee.

Part-time workers who have been on the job for two months or less are exempt from registering in the program. But if they work more than 75 percent of the hours put in by a full-time worker, the employer must remove them from the national health insurance or pension program and put them on the social insurance framework.

The Board of Audit probed the working standards of about 700 post offices nationwide during the first six months of the year. It found that in roughly 300 of them, several hundred workers who had been hired as part-timers were working shifts similar to those of full-time employees.

According to the Posts and Telecommunications Ministry, these part-timers assist in sifting through and delivering mail.