Abstract: In the injection molding of a hollow shaped article by the injection of not only molten synthetic resin but also compressed gas into a die, a method is provided by this invention which effects the supply of the compressed gas with a multistage compressor and the injected compressed gas is recovered in a recovery container connected to the suction side of the multistage compressor. For the injection molding of a hollow shaped article by the injection of not only molten synthetic resin but also compressed gas into a die, an apparatus is provided by this invention which has a gas nozzle for injecting the compressed gas into the die connected through a first switching valve to the discharge side of the multistage compressor and, at the same time, has the gas nozzle connected through a second switching valve to a recovery container connected to the suction side of the multistage compressor.