A method for producing foamable injection molded product using an injection apparatus, wherein the injection apparatus comprises: a plasticizing unit that plasticizes a raw material resin, an injection unit that injects a plasticized molten resin, and a connection part having a rotary valve in its inside; wherein the plasticizing unit is provided with a heating cylinder having a foaming gas injection valve, a hopper, for feeding the pelletized raw material resin to the heating cylinder, provided on the rear end side of the heating cylinder, and a screw arranged inside the heating cylinder, the injection unit is provided with a heat-retaining cylinder that a nozzle part to be attached to a mold is connected to the front end side thereof through the connection part, and the front end sides of the heating cylinder and the heat-retaining cylinder that are arranged in parallel to each other are connected through the connection part, the method comprises: a plasticizing process wherein the raw material resin is plasticized by means of the screw provided inside the heating cylinder of the plasticizing unit, the foaming gas injected from the foaming gas injection valve is dissolved in the plasticized molten resin, characterized in that the screw retreats according as the molten resin accumulates in the front end side of the heating cylinder by the screw that is rotating, a transfer process wherein the nozzle part is closed and the plasticizing unit and the injection unit are connected by switching the rotatory valve, the screw that has retreated to a predetermined position is caused to advance while rotating, and the molten resin which has been accumulated in the front end side of the heating cylinder is transferred to the injection unit; and an injection process, wherein the plasticizing unit is closed and the injection unit and the nozzle part are connected by switching the rotatory valve, the molten resin is injected from the injection unit; wherein, when the transfer process is completed, the plasticizing process re-starts without stopping the rotation of the screw and the injection process is conducted, and when the injection process is completed, the plasticizing process is completed and the transfer process re-starts. a plasticizing unit that plasticizes a raw material resin, an injection unit that injects a plasticized molten resin, and a connection part having a rotary valve in its inside; wherein the plasticizing unit is provided with a heating cylinder having a foaming gas injection valve, a hopper, for feeding the pelletized raw material resin to the heating cylinder, provided on the rear end side of the heating cylinder, and a screw arranged inside the heating cylinder, the injection unit is provided with a heat-retaining cylinder that a nozzle part to be attached to a mold is connected to the front end side thereof through the connection part, and the front end sides of the heating cylinder and the heat-retaining cylinder that are arranged in parallel to each other are connected through the connection part, the method comprises: a plasticizing process wherein the raw material resin is plasticized by means of the screw provided inside the heating cylinder of the plasticizing unit, the foaming gas injected from the foaming gas injection valve is dissolved in the plasticized molten resin, characterized in that the screw retreats according as the molten resin accumulates in the front end side of the heating cylinder by the screw that is rotating, a transfer process wherein the nozzle part is closed and the plasticizing unit and the injection unit are connected by switching the rotatory valve, the screw that has retreated to a predetermined position is caused to advance while rotating, and the molten resin which has been accumulated in the front end side of the heating cylinder is transferred to the injection unit; and an injection process, wherein the plasticizing unit is closed and the injection unit and the nozzle part are connected by switching the rotatory valve, the molten resin is injected from the injection unit; wherein, when the transfer process is completed, the plasticizing process re-starts without stopping the rotation of the screw and the injection process is conducted, and when the injection process is completed, the plasticizing process is completed and the transfer process re-starts. a plasticizing process wherein the raw material resin is plasticized by means of the screw provided inside the heating cylinder of the plasticizing unit, the foaming gas injected from the foaming gas injection valve is dissolved in the plasticized molten resin, characterized in that the screw retreats according as the molten resin accumulates in the front end side of the heating cylinder by the screw that is rotating, a transfer process wherein the nozzle part is closed and the plasticizing unit and the injection unit are connected by switching the rotatory valve, the screw that has retreated to a predetermined position is caused to advance while rotating, and the molten resin which has been accumulated in the front end side of the heating cylinder is transferred to the injection unit; and an injection process, wherein the plasticizing unit is closed and the injection unit and the nozzle part are connected by switching the rotatory valve, the molten resin is injected from the injection unit; wherein, when the transfer process is completed, the plasticizing process re-starts without stopping the rotation of the screw and the injection process is conducted, and when the injection process is completed, the plasticizing process is completed and the transfer process re-starts.