Double Bed (1983)

Kato is a small time TV producer. He has a wife Masako, and a young son Taro. Kato also has a friend Yamazaki who he knew since college. Yamazaki has a girl friend Riko who lives with her younger sister Yuko who's an actress. Yamazaki is a lyric writer/womanizer and he starts to have an affair with Masako, but he's still going out with Riko.