Koshino began learning judo with a local sports team while he was in elementary school, and became the Hokkaidō Prefecture champion for his weight class in junior-high school. He placed third in the inter-highschool tournament during his senior year, and entered Tōkai University in 1985. He won the Shoriki Cup twice while attending the university, and joined the Toyo Suisan corporation after graduating. He won the Kodokan Cup for 4 consecutive years from 1989–1992 and All-Japan Weight Class Judo Championships for 3 consecutive years from 1989–1991, and placed second in the World Judo Championships in 1989 before winning the world championships in 1991. He retired from competitive judo after finishing with a bronze medal at the 1992 Summer Olympics. He became an instructor for the International Budo University in April, 1993.