Nagai was born in Saitama on April 16, 1952. After graduating from Saitama Urawa Minami High School, he joined Japan Soccer League club Furukawa Electric in 1971. The club won the league champions in 1976 and 1985/86. The club also won 1976 Emperor's Cup, 1977, 1982 and 1986 JSL Cup. He retired in 1988. He played 272 games and scored 63 goals in the league. It is the record for most appearances in Japan Soccer League Division 1. In 1976, he was selected "Japanese Footballer of the Year" awards. He was selected Best Eleven 5 times.