Muto went to war in China flying with the 12th Air Group. He earned his first air victory on 4 December 1937 during the Battle of Nanking when he shot down a Soviet-made Polikarpov I-16. Muto continued fighting in China, flying many sorties over Hankou to become an ace with five victories. For his distinguished service, he was honored with an official commendation on 30 April 1938. Among his fellow pilots he was well-liked for his good sense of humor.