Opposition lawmaker Akira Ishii, who is suspected of fraud involving secretary pay, is believed to have cheated the government out of about ¥8 million ($54,000), informed sources said Thursday.

The 68-year-old member of the House of Councilors, the upper chamber of parliament, from Nippon Ishin no Kai is suspected of receiving salaries for a state-paid public secretary who actually did not work for him.

On Thursday, the special investigation squad of the Tokyo District Public Prosecutor's Office continued to search places related to Ishii, including his office in the city of Toride, Ibaraki Prefecture.

The special squad has questioned several people concerned, including a former aide to the lawmaker, on a voluntary basis. It will investigate the details of the case, such as the flow of money, by analyzing materials confiscated through the raids.

After serving as a member of the Toride Municipal Assembly, Ishii was elected to the parliament for the first time in the 2009 election for the House of Representatives, the lower chamber, running from the now-defunct Democratic Party of Japan.

Ishii is currently in his second term as an Upper House lawmaker, after winning a seat on the chamber for the first time in the 2016 election, in which he ran from the then-Osaka Ishin no Kai.

Nippon Ishin leader Hirofumi Yoshimura said Thursday that Ishii should resign from parliament if the allegations are true.

"Such a thing must never happen," Yoshimura, also governor of Osaka Prefecture, told reporters at the prefectural government office in the city of Osaka, referring to the suspected fraud. "We have thoroughly implemented painful reform measures," he added.

According to Yoshimura, Nippon Ishin Secretary-General Hiroshi Nakatsuka contacted a lawyer for Ishii on Thursday. Nippon Ishin will question Ishii himself shortly about the alleged fraud, Yoshimura said, adding that the party will consider whether to punish him after confirming facts.

Japanese lawmakers are allowed to employ up to three secretaries on government payroll. In the wake of a series of fraud cases by lawmakers over secretary pay in the 1990s to 2000s, a relevant law was revised in 2004 to directly pay salaries to secretaries.

Earlier this year, however, former Upper House lawmaker Megumi Hirose was convicted for defrauding the government of some ¥3.58 million in secretary pay and retirement allowances.