To provide a synthetic resin molding which is joined by a recess portion and a projecting portion and does not easily break away after joining, a synthetic resin mold assembly, and a method of manufacturing a synthetic resin molding.

The diameter of a projecting portion 18 forming a joint portion of foamable synthetic resin is constituted to expand toward a tip end, and the inner diameter of a recess portion in which the projecting portion is inserted to fit with is constituted to expand toward a bottom portion. The foamable synthetic resin, when molded, is unmolded in a state of being still soft, in the mold, so that the resin is elastically deformed to extract a part corresponding to a shape of the recess portion and the projecting portion from the resin and then hardened.