The House of Councillors approved Thursday a request by Democratic Party of Japan legislator Kunihiko Muroi to be allowed to relinquish his seat at a plenary session.
Muroi, who was elected to the upper house on the DPJ's proportional representation list, aims to run on another party's ticket in the upcoming upper house election. The 66-year-old has already submitted a written resignation to the DPJ.
But the main opposition party, which lost power in a general election in December, urged him to give up his parliamentary seat rather than granting his plea to quit.
Muroi's resignation will leave the parliamentary group led by the DPJ intact with 85 seats in the 242-seat chamber, one more than a rival group led by the ruling Liberal Democratic Party, as Kanako Otsuji, who ranks next on the DPJ list, will take over the vacated seat possibly in late May.
Otsuji, a 38-year-old former Osaka prefectural assembly member, is known for being Japan's first openly lesbian politician.
The DPJ had planned to endorse Muroi as a proportional representation candidate for the upper-house vote expected in July.
But he sought to leave the party on April 19 on the grounds it had lost room for politicians with conservative ideals. He has not specified the party with which he wants to run next.
He was first elected to the House of Representatives in 2003, but failed to return in the 2005 general election. He regained a Diet seat in the 2007 upper house election.
He was parliamentary secretary of the Ministry of Land, Infrastructure, Transport and Tourism from September 2011 to October 2012 in the Cabinet of Prime Minister Yoshihiko Noda.