He was a major figure in fandom in Japan and contributed to establishing the Japanese science fiction genre.

A native of Kanazawa, Ishikawa, in 1957 Takumi started Japan's first successful science fiction fanzine Uchūjin initially published monthly; many contributors to the fanzine later became pro, including Shin'ichi Hoshi, Sakyo Komatsu, Ryu Mitsuse and Yasutaka Tsutsui, and formed the first generation of modern Japanese science fiction authors.

[2] He worked on the formation of the Federation of SF Fan Groups of Japan, founded in 1965, and served as its chair from 1966 through 1970.

[4] Under the pen-name Rei Kozumi (小隅 黎, Kozumi Rei), a play on "cosmic ray,"[4] he translated as many as sixty science fiction novels from English into Japanese, including E. E. Smith's Lensman series and Larry Niven's Known Space series.

Also as Rei Kozumi, he wrote three children's books, Superhuman ‘Plus X’ (1969), Operation Moonjet (1969), and Revolt in North Pole City (1977), and was also principal author of The World of Popular Literature (1978).