After his studies in literature at Shinshu University, he began work as a director's assistant.

8 received widespread acclaim for tackling the issue of a woman forced into prostitution in Borneo before the outbreak of World War II.

Kinuyo Tanaka won the Best Actress Award at the 25th Berlin International Film Festival for her performance.

[6] Kumai's follow-up film was 1976's Cape of North, starring French actress Claude Jade as a Swiss nun who falls in love with a Japanese engineer on a trip from Marseilles to Yokohama.

[8] Other works include Death of a Tea Master starring Toshirō Mifune as Sen no Rikyū (Silver Lion at the 46th Venice International Film Festival), and the 2002 film The Sea Is Watching, based on Akira Kurosawa's posthumous script.