From the age of 3, Nagai spent five years in Ipatinga, Brazil, where he started playing street football with his childhood friends.

[2] He joined the Fukuoka University club in April 2007 and won the 33rd Prime Minister's Cup All Japan College Soccer Tournament in 2009.

[3] On 23 December 2009, he was named in the squad for Japan's 2011 AFC Asian Cup qualification against Yemen,[4] and made his full international debut for the team in the fixture on 6 January 2010.

[6] He was transferred from Nagoya to Standard Liège of Belgian Pro League in January 2013, but was loaned back to Nagoya from August 2013 to June 2014, due to limited playing opportunities at Standard Liège.

In January 2015, the loan deal was terminated, and Nagai returned to Nagoya on a permanent basis.