In March 1943, Mochizuki was appointed division leader (buntaichō) in the newly-formed 281st Air Group that was stationed in the Kuril Islands. The same year he was transferred to Roi in the Marshall Islands. After USN carrier strikes destroyed most of the IJN aircraft in the Marshalls, he became stranded on Roi. Mochizuki disappeared and was presumed killed during the American invasion on 6 February 1944.