Itoi was drafted into the Fighters in the fall of 2003, and was considered a power pitcher, being able to pitch 151 km/h (94 mph) fastball but has control problems and were unable to muster up decent breaking balls.

Although Itoi had problems developing as a good pitcher in the pros, his outstanding batting sense, running speed (50m in 5 seconds), and strong throwing arm prompted Fighters GM Shigeru Takada to convert him into an outfielder.

He ended the year hitting .306 with 15 home runs, 24 stolen bases, and a league high 40 doubles, and won his first Golden Glove award.

Itoi continued to be an integral part of the Fighters, winning his second Golden Glove in 2010 and ended the year hitting .309 with 15 home runs and 26 stolen bases.

After the NPB switched the official game ball into the pitcher friendly ball that lead to a league-wide fall in batting statistics, Itoi continued to flourish in the 2011 season, hitting .319 with 11 home runs and 31 stolen bases, and a league high .411 OBP (the only player with OBP of more than .400 that year in whole of NPB) mainly as the third batter.

He started to pick up in the second half of the season, hitting .380 with 3 home runs in September to win the Monthly MVP award.

Itoi expressed interest at the end of the season to be posted to the Major Leagues, but was instead traded to the Orix Buffaloes in a 5 player deal on January 25, 2013.

Itoi was involved in a surprising 5 player trade at the end of the 2012 season, which saw him moving to Pacific League rivals Orix Buffaloes.