Kaji was a member of the Japan U-20 national team for the 1999 World Youth Championship. He played 3 matches and Japan won the 2nd place. On October 8, 2003, Kaji debuted for the Japan national team against Tunisia. He became a regular player as right side back from 2004. In 2004, he was selected Japan for 2004 Asian Cup and Japan won the champions. He also played at 2006 World Cup and 2007 Asian Cup. However his opportunity to play decreased behind young player Atsuto Uchida in 2008. He played 64 games and scored 2 goals for Japan until 2008.