An injection molding apparatus includes a forming mold provided with a gas inlet through which a gas is introduced into an area between a rear molding surface in a cavity and a resin material inside the cavity. A plurality of the gas inlets is provided along a passage direction of the resin material in the cavity. In injection molding the resin molded article, a molten resin material is introduced into the cavity from a resin inlet and, when filling the inside of the cavity with the resin material, the introduction of the gas is sequentially started, beginning with a gas inlet provided in an area inside the cavity which is first filled with the resin material, thereby sequentially pressing the relevant resin material against a front molding surface of the cavity.