Kaji made his professional debut in the J-League’s top division with Cerezo Osaka on July 25, 1998. Two seasons later, the defender had a brief stint with Oita Trinita of the Second Division, before returning to Division 1 with FC Tokyo in 2002. Two years into his stint with the country’s capital team, Kaji celebrated his first championship by scoring the winning penalty kick against Urawa Red Diamonds for the J-League Cup trophy.
In 2006, the Minamiawaji native signed with Gamba Osaka, where his 39 appearances and two goals earned him a spot on the J-League Best IX. Since then he continued to be an undisputed regular with the club, registering a total of 225 appearances.
At the international level, Kaji moved up through Japan’s national youth ranks and was part of the squad that finished runner-up in the 1999 FIFA World Cup Youth Championship at age 19. Five years later, the defender was called up to the Senior Squad, earning his first cap on Oct. 8, 2003 in a friendly against Tunisia. Since then, Kaji recorded 64 caps and competed in the 2006 FIFA World Cup in Germany, as well as the 2003 and 2005 Confederations Cup. Additionally, Kaji started in all six matches to help Japan conquer the 2004 AFC Asian Cup.