When molding of a heat-curable resin such as unsaturated polyester resin and the like is conducted by means of the casting method, concavities in the molded product generated as a result of the curing shrinkage of this resin are prevented. A central core member comprising ABS resin or the like is disposed in advance within mold, a heat-curable resin liquid is injected thereinto, and heated, and at this time, an amount of expansion in the volume of central core member is caused which corresponds to the amount of volume shrinkage resulting from curing of the heat-curable resin, and the expansion of the central core member is synchronized with the curing shrinkage of the heat-curable resin.