Nagai was a member of the Japan U-20 national team for the 1997 World Youth Championship. He played all 5 matches and scored 1 goal. He also represented U-20 Japan at the 1999 World Youth Championship. He played all 7 matches and scored a goal in the semifinal against Uruguay. Japan won the 2nd place. On April 16, 2003, he debuted for the Japan national team against South Korea and scored a goal in this match. He was also selected Japan for 2003 Confederations Cup. He played 4 games and scored 1 goals for Japan in 2003.