His relatives were resolved that he should become a Buddhist priest, but he moved to Tokyo at the age of 15 in order to find work, and to pursue a literary career.

In 1912, Kasai joined Shigeo Funaki and Kazuo Hirotsu in founding a new literary magazine, Kiseki ("Miracle"), and contributed various works to it.

His major works include Kanashiki chichi ("Mourning Father", 1912), Akuma ("Devil", 1912), and Kohan Nikki ("Lakeside Diary", 1924).

Kasai lived in Kamakura, Kanagawa for many years, due to the presence of many fellow writers, and because the sea air was considered healthier for his weak lungs.

He died in Tokyo at the age of 41 after a long struggle against tuberculosis aggravated by stress over the collapse of his marriage due to an affair.