Kan, under fire for his handling of the earthquake, tsunami and nuclear plant crisis this spring, announced his resignation Aug. 10.
The field is the largest in Japanese electoral history, The Japan Times reported. But the decision could be made by Ichiro Ozawa, a power in the party who is under indictment for alleged fundraising irregularities.
The 398 Democratic Party members in the Diet are scheduled to vote Monday. Ozawa has a bloc of 120 votes, the Times said, and supports Trade Minister Banri Kaieda.
The other leading contenders are Finance Minister Yoshihiko Noda and former Foreign Minister Seiji Maehara, both considered to be foes of Ozawa. Farm Minister Michihiko Kano and ex-Transport Minister Sumio Mabuchi also are in the mix.