Inagaki was born in Osaka, moved to Akashi in Hyōgo Prefecture while he was in elementary school, and spent much of his childhood in Kōbe. He graduated from Kwansei Gakuin Junior High School. In 1968 he won the first annual Japan Literature Grand Prize for Shōnen'ai no Bigaku(少年愛の美学,, The esthetics of boy-love?), an essay on "aesthetic eroticism", where he divides stories into A (anal), V (vaginal), P (penile) and K (clitoral) varieties and "describe[s] the historical, psychological, and metaphysical ramifications of the love of beautiful boys in an eclectic blend of ideas culled from history, Freudianism, pop psychology, and existentialism." 
Inagaki's works often dealt with themes including flight, astronomical objects, and erotic and romantic relationships among beautiful adolescent boys. His stories on the later topic, and his essays in Shōnen'ai no Bigaku, were an influence on early writers of the yaoi genre such as Keiko Takemiya.