Diet member Shigeyuki Goto submitted a letter of resignation Wednesday to the Democratic Party of Japan.
Goto, 47, is a former Finance Ministry bureaucrat elected to the House of Representatives from Nagano Prefecture's No. 4 district.
He had indicated earlier this month that he would leave the main opposition party due to policy differences, according to informed sources. He suggested he would become an independent, they said.
With his supporters reportedly urging him to join the Liberal Democratic Party, however, he has not ruled out a maneuver of this kind.
His departure deals another blow to the DPJ, which in late December saw five members defect to join forces with members of the revamped New Conservative Party, the smallest party in the tripartite coalition, after the LDP and New Komeito.
Goto indicated he has no plans to join the NCP.