A molding method of a resin molded product in which it is possible to increase the welding strength between the core member and the resin sheets. In a molding method of a resin molded product in which a core member including a foaming element is interposed between at least two resin sheets in a melted state into a laminated product, wherein the resin sheets include fibrous filler, and while melting surfaces of the core member by heat of the resin sheets, the core member and the resin sheets are welded onto each other, and the core member and the resin sheets are clamped by a metallic mold, to mold the resin molded product.