He competed for his native country at the 2004 Summer Olympics in Athens, Greece, where he was stopped in the first round of the men's light flyweight division (– 48 kg) by Ethiopia's Endalkachew Kebede (21:26).
[3] 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.
[5] Igarashi defeated Mexico's Wilbert Uicab in the WBC flyweight title eliminator via a unanimous decision at the Yoyogi National Stadium Second Gymnasium on November 6, 2011.
[6] On July 16, 2012, Igarashi barely outpointed Sonny Boy Jaro via a split decision to win the WBC, The Ring and lineal flyweight titles in Kasukabe, Saitama.