The invention provides a molding apparatus adapted to use a screw having an overall length permitting the use of an existing molding machine cylinder as it is, and is capable of realizing a gas impregnation step. The screw 3 for use in the gas impregnation step comprises a resin melting portion C disposed at an upstream position for rendering the resin molten, a molten resin nonfilled-up portion E integral with the downstream side of the resin melting portion C for making the resin lower than an inert gas at a gas supply opening 5 in pressure, and a gas impregnation portion D integral with the downstream side of the molten resin nonfilled-up portion E for supplying the inert gas and impregnating the molten resin with the inert gas.