PURPOSE: To secure dimensional accuracy by carrying out molding with the back side of a liner being pressed by means of a pressing pin and water-tightly closing a hole produced by said pressing pin, when molding the casing of a volute pump with a synthetic resin, together with its liner, with a synthetic resin.

CONSTITUTION: A liner 22 is pressed by a pressing pin 26A provided on a movable mold 26, and a casing cover 4 is molded with a high-pressure molten resin from a gate 26B, together with the liner 22 in an integrated form. A hole produced by the pressing pin 26A taken out from the mold is water-tightly closed by means of ultrasonic welding, etc. Thereby, a pump which is excellent in corrosion resistance and wear resistance and which can secure dimensional accuracy, without leakage of water, can be manufactured.