Two judges scored the fight 116-112 and 115-113 for Japanese Igarashi, while the other gave it 116-112 for the defending champion from the Philippines.
"It was the first time that I received a series of heavy punches like his. I won it because of my mentality, not because of my technique or physical strength," said Igarashi.
"I don't care how many times I can defend the title. I just want to become a boxer who can say I'm the world champion, with my chest out."
On the same bill Takashi Uchiyama retained his WBA super-featherweight title after his fight against Filipino Michael Farenas ended in a technical draw when the two fighters clashed heads early in the third round, resulting in a cut above Uchiyama's eye.
"I'm very sorry for my fans because of such a poor fight," said Japan's Uchiyama.