Yaegashi (17-3) won by judges' scores of 117-108, 116-109 and 115-110.
It was just the second title defense by Igarashi (17-2-1).
In another WBC title match, Takashi Miura also beat a champion. Miura scored a ninth-round TKO over Gamaliel Diaz.
Miura is 25-2-2 and Diaz is 37-10-2.
2014 amfAR Inspiration Gala [PHOTOS]
Julius Randle breaks leg during NBA debut