Kosai majored in philosophy at Tokyo's Sophia University, before becoming interested in film.
In 2005, he won a competition for the short-film showcase "Remarkable 5" and was selected as a director from Japan. Though it was his directorial debut, the short "RIGHT PLACE" was highly regarded by Shots, selecting him as "Shots New Directors."
Kosai then went on to win awards at 'Raindance Film Festival' (Diesel Film of the Festival Award), 'New York City Short Film Festival' (Best Foreign Film Award) and 'ResFest' (World Tour Selection Award).
Most recently he directed the Adidas spot ‘ Break Up service’ for TBWA, London. He is featured as ‘ Director to Watch’ in Boards Magazine, December 2008.