Foamable injection moldings are manufactured by performing a plasticizing process, a transfer process, and an injection process, and repeating said processes. In the plasticizing process, a plasticized molten resin impregnated with a foaming gas is fed to the tip of a heating cylinder (20) by a screw (21) rotating inside the heating cylinder (20) and the screw (21) moves backward as the molten resin accumulates in the tip of the heating cylinder (20). In the transfer process, the screw (21) moves forward while rotating to deliver the molten resin accumulated in the tip of the heating cylinder (20) from the plasticizing unit (2) to an injection unit (3). The injection process injects the molten resin from the injection unit (3). As a result, when injection-molding a foamable injection molding impregnated with foaming gas, plasticization of the material resin during the forming cycle and impregnation of the foaming gas into the plasticized molten resin are improved and productivity can be improved.