PURPOSE: To make a molding of a synthetic resin lightweight, to shorten the molding time, and to lower the cost when the molding is produced, by separately carrying out a step of molding a core section having holes that opens to the outer surface and whose inside cross-sectional area is larger, and a step of molding an outer shell section on the outside of the core section.

CONSTITUTION: When a mixture of a polyol, a foaming agent capable of foaming upon heating, and an isocyanate is injected into a cavity 12, the polyol reacts with the isocyanate and a core section 6 having a number of cells therein is molded. In this case, as the tips of pins 17 abut against the mold surface of the cavity 10, the core section 6 is formed with holes 7 whose cross-sectional area is larger toward the inside and that extend from the outer surface thereof to an attaching surface 7. Then the core section 6 is placed in a cavity 13, and a resin is injected and is allowed to enter the holes 7 so that the outer shell section 8 can be adhered firmly to the core section 6.