帝拳プロモーション
After turning professional in 2006, he made his professional debut with a third-round knockout on August 13 of that year. Fighting out of Teiken Boxing Gym in Tokyo under Yūichi Kasai's instruction, Igarashi gained the Japanese flyweight interim title via a split decision on August 2, 2008. Although Igarashi lost to the regular champion Tomonobu Shimizu four months later, he captured the vacated Japanese flyweight title by a third-round knockout on February 5, 2011. He defended it once, until June of the same year.