Investigators searched the Diet office of Kanju Sato on Friday for evidence over allegations that he misappropriated a secretary's state-paid salary.
Aichi Prefectural Police also searched Sato's room in the House of Representatives dormitory, investigators said.
Sato, 62, tendered his resignation from the Democratic Party of Japan on Wednesday and the Lower House of the Diet on Thursday. He is suspected of registering a woman as a secretary between June 2000 and last April and personally receiving her state-funded salary even though she never worked for him.
The 51-year-old secretary's husband worked for Sato's fund-management organization.
Police will question Sato and his wife, the sources said. Sato's 52-year-old wife is a secretary to the lawmaker and her salary is paid by the state.
Police sources said Sato deposited the money in a bank account opened by a DPJ chapter in Aichi Prefecture. Sato heads the chapter and it oversees his electoral district in the prefecture.
They said police believe Sato amassed the money for political activities, and will try to prove this with evidence seized in the search.
Sato's wife managed the local chapter's bank account. While some of the money was used to run Sato's office or pay the salaries of other personnel, most of it remains in the account, they said.
Some of the woman's pay was allegedly deposited into an account under the name of Sato's wife.
The Diet on Friday approved Sato's resignation at its plenary session.