To perform processing regardless of the thinness by removing a salient boss molded integrally at the external surface of a casing in die-cast molding of the casing.

A generally circular plate casing 1 is molded by aluminum die casting. For this casing 1, a tubular salient boss 2 tubular as a whole is formed by integrated molding at the time of die cast molding, and this salient boss 2 is made at the external face 1a of the casing 1, and the inner face 1 is subjected to finishing processing by after processing. This way, the casing 1 is molded with an aluminum die cast mold, and then it is chucked with the chuck of a machine tool, and the after processing of this casing 1 is performed into a given accuracy, and then this salient boss 2 is removed by a processing machine. Hereby, the casing 1 can be molded into an optional thickness, and the flat motor can be thinned.