An apparatus and method for producing a tank for a heat exchanger. The apparatus includes molds which form the tank such that it has an open end, an annular skirt about the entire periphery of the open end, and an annular groove in the annular skirt. A space in the molds receives the skirt, and a projection on one of the molds urges the outer and inner side surfaces of the skirt toward each other after a resin has been injected into the annular groove. A guide surface extends from the bottom of the projection to a position substantially even with a bottom surface of the annular groove to govern the amount of deformation of the side surface which is urged by the projection. A bubble removal element, such as a protuberance or groove, is formed in the annular groove at a position where opposite ends of the resin impinge on each other when injected from an injection gate, to prevent bubbles from forming in the resin.