[4][5] His father worked as an engineer at a telecom company; his mother was employed by a kimono shop.

[4][6] After a succession of jobs including being a roadie, a waiter and a construction worker, he started a publishing business.

[6] Kaji launched a quarterly puzzle magazine in 1980 called Nikoli,[7] together with two friends from his childhood.

[4][10] He shortened it from Suuji wa dokushin ni kagiru ("numbers should be single") at the urging of his fellow workers.

[4] Kaji died on 10 August 2021 at his home in Tokyo at age 69, from bile duct cancer.