To solve the problem wherein a conventional trial for heating and melting the surface of foam to form a non-foamed layer causes the complication of a molding method, a necessary or above time is consumed in a heating process and a cooling process, there is a problem in the smoothness, thickness and uniformity of an obtained surface hardened layer and an industrial demand is not satisfied heretofor.

A mold having both of a steam blow hole and a space part permitting heating and cooling media to flow is used not only to perform a preheating process for heating, foaming and molding a foamable synthetic resin by flowing steam into the mold and a post-heating process for allowing a heating medium, which is heated to the melt temperature or above of the foamable synthetic resin to flow the space part while foaming pressure to +0.04 MPa or above. Thereafter, a cooling medium is allowed to flow to the space part to cool the mold and cooling water is sprayed on the mold if necessary to cool the mold to stop not only the surface melting of the foamable synthetic resin but also the advance of foaming thereof. This cooling process is performed to manufacture the synthetic resin foam having a surface hardened layer.