Nagai was born in Fukuyama on March 5, 1989. When he played for Fukuoka University, he also played for Avispa Fukuoka (2009) and Vissel Kobe (2010). After graduating from the university, he joined J1 League club Nagoya Grampus in 2011. He played many matches from first season and Nagoya won the 2nd place in the 2011 J1 League. In January 2013, he moved to Belgian Pro League club Standard Liège. However there were few opportunities to play. In August 2013, he returned to Nagoya. Nagoya was relegated to J2 League end of the 2016 season. In 2017, he moved to FC Tokyo.