A metal mirror manufacturing method effectively uses the precision of a polygon mirror blank obtained by press working by omitting a peeling step, thereby improving the productivity. In order to achieve this object, there is provided a method of manufacturing a polygon mirror by finishing each surface of a metal mirror blank having undergone press working into a mirror surface by a cutting process, including the steps of cutting intersecting portions between the respective surfaces of the mirror blank by a cutting process by a predetermined amount, and flattening each of the surfaces by causing a mirror-working cutting tool to enter the mirror blank from a cut mark of each cut intersecting portion.