The present invention applies to a molding method for a fiber-reinforced plastic structure having an internal cavity. Firstly, grain groups, which mainly consist of a plurality of high-rigidity grains, are accommodated in bags, and a plurality of cores are formed. A reinforcing fiber substrate, is placed between the plurality of adjacent cores so as to be interposed therebetween. For example, a plurality of molding base materials are prepared by surrounding each core with a prepreg, and the plurality of molding base materials are combined and placed inside a molding die, and the molding base materials are compression molded. When compression molding, a part of the outer surface of the cores is locally pressurized, and the internal pressure of the cores is increased, changing the shape thereof, thus eliminating voids that are present between the cores and the prepreg and/or the prepreg and the molding surface of the die.