The small opposition Social Democratic Party picked on Monday the chairman of its policy board as its new leader.
Tadatomo Yoshida, a first-term member of the House of Councillors, was elected to succeed Mizuho Fukushima, who resigned in July to take responsibility for its dismal showings in parliamentary elections.
Yoshida, 57, beat Taiga Ishikawa, a 39-year-old assemblyman in Tokyo's Toshima Ward, in a two-candidate race, with voting by the 17,410 party members including five lawmakers.
Yoshida garnered 9,986 votes and Ishikawa 2,239 votes.
Yoshida, who will serve the remainder of the term of Fukushima, 57, and stay in his post until the party's convention in the early part of 2016, is expected to mostly retain the executive lineup including secretary general Seiji Mataichi.
Yoshida had argued for a departure from the party's dependence on labor unions and called for promoting cooperation with civic groups and local communities.
His rival Ishikawa had vowed to rebuild the party's power by appointing young people to the party leadership.
In last December's lower house election, the SDP lost three of its five seats and failed to stop the downward momentum by losing one of its two contested seats in the July upper house poll, in which the ruling coalition led by Prime Minister Shinzo Abe won a sweeping victory.
It is the SDP's first leadership election since 1996 when the party changed its name to the current one. All leadership races in the past were held without a contest.
Yoshida, who is from Oita Prefecture and had served as an Oita prefectural assembly member, was first elected to the upper house in 2010.