Upper House member Kunihiko Muroi submitted his resignation to the Democratic Party of Japan on Friday, making him likely to become the fourth member to quit since it was ousted in December's general election.
His departure will leave the main opposition party with 84 of the 242 seats in the House of Councilors — only one more than the ruling Liberal Democratic Party.
If the LDP's candidate wins the April 28 by-election for an Upper House seat in Yamaguchi Prefecture, the two parties will have equal strength in the chamber.
"Conservative politicians have lost the opportunity to demonstrate our abilities in the DPJ," Muroi, 66, said.
Muroi hinted that he will likely run on another party's ticket in July's House of Councilors election.
Earlier this year, the DPJ expelled Upper House lawmakers Tatsuo Hirano, a former reconstruction minister, and Minoru Kawasaki after they submitted formal letters of resignation to the party.
Another DPJ lawmaker in the chamber, Emiko Uematsu, submitted a letter of departure in February but has been allowed to remain in the DPJ-led parliamentary group.