A method of manufacturing a fiber-reinforced plastic includes placing a compressible reinforcing fiber base material made of discontinuous reinforcing fibers in a cavity of a mold, injecting melted thermoplastic resin into the cavity, and impregnating the resin into the reinforcing fiber base material, wherein the mold is adapted to change a cavity volume of the mold, and the reinforcing fiber base material after having been or while being impregnated with the thermoplastic resin is compressed in the cavity by reducing the cavity volume of the mold after injecting the melted thermoplastic resin.