Nippon Ishin no Kai, which saw its seat count nearly quadruple in the Lower House election, decided to keep Ichiro Matsui on as leader after deciding not to hold a presidential election
With the election seen to be a close contest, the four candidates are aiming to attract as many votes as possible.
Whether or not to cooperate with the Japanese Communist Party will be a top issue for the next leader of the nation's top opposition party.
Lawmakers backing constitutional amendments now account for more than two-thirds of the seats in the Lower House.
The big winner appeared to be the party’s largest faction, now headed by former Prime Minister Shinzo Abe, while the one led by Prime Minister Fumio Kishida also strengthened its presence.
Seiji Osaka, a former special adviser to the prime minister, and Chinami Nishimura, a former senior vice minister of health, labor and welfare, are also expected to run for the post.