Born on June 1, 1930, in Nagano Prefecture, Kumai attended Shinshu University. After graduation, he became an assistant director on independent films before joining Nikkatsu Studios in 1954 where he continued working as an AD and screenwriter.
He made his directing debut in 1964 with The Long Death (Teigin Jiken: Shikeishu), based on the case of a mass cyanide poisoning at a Tokyo bank. Kumai soon gained fame for making bold films with strong social messages.
His third film, the big-budgeted Sun Over Kurobe Gorge (Kurobe No Taiyo) starred and was co-produced by Toshiro Mifune and Yujiro Ishihara (late brother of current Tokyo Governor Shintaro Ishihara). After the film's commercial success, Kumai left Nikkatsu Studios in 1969 and remained independent.
In 1974 he directed Sandakan 8 (Sandakan Hachibanshokan Bokyo), which received an Oscar nomination for best foreign-language film and a Silver Bear in Berlin for star Kinuyo Tanaka's performance. The film depicted the lives of karayuki-san, Japanese women forced to work overseas as prostitutes after the Meiji Restoration.
In 1987, Kumai received the Special Jury Prize in Berlin for The Sea And Poison (Umi To Dokuyaku), based on Shusaku Endo's famous novel.
Feudal drama The Death Of A Teamaster (Sen No Rikyu), again starring Mifune, won the Silver Lion in Venice in 1989. Kumai's films also received awards and nominations at festivals including Montreal, Moscow and San Sebastián.
Kumai was also known for his controversial remarks. He was outspoken against famed directors such as Yasujiro Ozu, whom he claimed was a slave to the studio system. Kumai continued to make films with hard-hitting messages, gaining him a respected and unique position in the Japanese film industry.
In 2001, Kumai received the Berlinale Camera prize, which the festival awards filmmakers it has long-running relationships with. Screened at Berlin that same year, Darkness In The Light (Nippon No Kuroi Natsu - Enzai) depicted the controversy behind the Matsumoto sarin case.
In 2002 he directed his final film, Umi Wa Miteita, based on an unmade screenplay by Akira Kurosawa.
Kei Kumai is survived by his wife and daughter.