Prosecutors on Friday questioned DPJ lawmaker Joji Yamamoto on suspicion of illegally using 20 million yen of his secretary's salary for personal use, investigative sources said.

Yamamoto, 37, is suspected of taking the money out of 30 million yen in salary paid to the secretary in charge of policy from October 1996 to September 1999, the sources said.

Lawmakers' government-appointed secretaries in charge of policy affairs receive between 7.5 million yen and 12 million yen a year from state coffers based on their age and experience, which is usually paid directly into their bank accounts. Yamamoto, however, reportedly paid the secretary between 50,000 yen and 100,000 yen a month, which he was able to do because he controlled the secretary's bank account.

The House of Representatives member allegedly used the money for purposes including paying his office rent.

Yamamoto has allegedly used his current secretary's salary in the same manner.

One of Yamamoto's secretaries reportedly told a third party that he or she was unaware of the real size of the salary until seeing a paper showing how much tax had been deducted, which in turn led to suspicion of Yamamoto's actions.

Tsutomu Hata, secretary general of the Democratic Party of Japan, told reporters at party headquarters Friday that he is sorry the allegation has arisen.

"The party leadership must apologize to the public as we failed to give thorough instruction to party members," he said.

Yamamoto, a former secretary to top DPJ policymaker Naoto Kan, was first elected to the Lower House in 1996 and re-elected in the June 25 general election.

In a seemingly similar case, Yojiro Nakajima, 41, a former Liberal Democratic Party Lower House lawmaker, was charged with defrauding the government of some 10 million yen paid by the state as salary for his purported policy secretary.

When the Nakajima case surfaced, Yamamoto reportedly urged the lawmaker not to relinquish his Diet seat.

Prosecutors plan to continue taking statements from people involved in the case, including Yamamoto, the sources said.