To improve the quality of a product and reduce the cost by forming a primary molding with the right molding and left molding forming an inflow port, a volute chamber, and an outflow port, and injection-forming a secondary molding on their outer surfaces.

The primary molding of the pump housing 300 of a hot-water circulating pump is constituted of the left molding 100 and right molding 101, a guide lug 109 is formed on the left molding 100 at a contact section with the right molding 101, and a guide groove 111 is formed at the portion of the right molding 101 corresponding to the guide lug 109 and is coupled with it. When the left and right moldings 100, 101 are assembled, the internal faces of the pump housing 300, i.e., an inflow port 105, an outflow port 103, and a volute chamber 107, are formed. A secondary molding 200 having the prescribed thickness is molded on the outer surfaces of the right and left moldings 100, 101 thus assembled, and the pump housing 300 is formed.