Seiko Noda, former posts and telecommunications minister, retained her House of Representatives seat Sunday.
Noda, 45, ran in the Gifu No. 1 district as an independent because the Liberal Democratic Party refused to let her run on its ticket after she voted against Prime Minister Junichiro Koizumi's postal privatization bills.
She will start her fifth term in the Lower House as an independent, though she is still an LDP member.
The party's leadership fielded Yukari Sato, 44, an economist and political newcomer, to run against Noda.
Sato won a seat in the proportional representation sector of the Tokai block, in which she was placed at the top on the LDP's candidate list.
The battle in the single-seat district attracted much attention as the local LDP chapter is one of several that defied an order from LDP headquarters not to back postal reform dissidents, including Noda.
Noda was also backed by the local chapter of the LDP's coalition partner, New Komeito.
"It was a very hard fight to win," said Noda in her victory speech. "I am grateful to my supporters, as you are the ones who brought me victory."
Other contenders in the district were Masanao Shibahashi, 26, a former bank employee fielded by the Democratic Party of Japan, and Satoru Ogawa, 52, from the Japanese Communist Party.