A manufacturing method according to an embodiment of this invention is a method of manufacturing a semiconductor device, which has: a first step of forming a first electrode 22 containing Ti or Ta on a top face of a nitride semiconductor layer 18; a second step of forming a second electrode 24 containing Al on a top face of the first electrode 22; a third step of forming a coating metal layer 26 covering at least one of an edge of a top face of the second electrode 24 and a side face of the second electrode 24, having a window 26a exposing the top face of the second electrode 24 in a region separated from the foregoing edge, and containing at least one of Ta, Mo, Pd, Ni, and Ti; and a step of performing a thermal treatment, after the third step.