Nagai was born in Shinjuku, Tokyo on February 14, 1979. He joined J1 League club Urawa Reds in 1997. He was loaned out to German 2. Bundesliga side Karlsruher SC from 1998 to 1999. He played 21 league games and scored 4 goals for reserve team. In 2003, Nagai took over the number "9" jersey from iconic Masahiro Fukuda after the latter retired from the game. In 2006, Urawa won the champions in J1 League which is first J1 champions in the club history. In 2007, Urawa won first Asian title AFC Champions League. Urawa also won the 3rd place at 2007 Club World Cup. In 2009, he moved to Shimizu S-Pulse and played for the club in 3 seasons. After that, he played for Yokohama FC (2012–13), Arterivo Wakayama (2014) and Thespakusatsu Gunma (2015–17).