PURPOSE: To easily control the wall thickness of a molded product and the forming position of a reinforcing rib by a method wherein a thermoplastic resin is injected into a mold cavity as a raw material in a molten state and the volume of the mold cavity is expanded while or after compressed gas is introduced into the injected molten resin under pressure to provide a large hollow part.

CONSTITUTION: An injection molding apparatus is set to a clamping state to form a mold cavity 9. A molten resin is injected into the mold cavity 9 and compressed gas is introduced into the molten resin under pressure from a compressed gas inlet 4. The volume of the mold cavity 9 is expanded after or while the compressed gas is introduced into the molten resin under pressure. At the time of the expansion of the volume of the mold cavity 9, a movable core 3 is allowed to retreat by the resin injection pressure remaining in the molten resin injected into the mold cavity 9, the pressure of the compressed gas and the operation of the low oil pressure supply means provided to the movable plate moving hydraulic mechanism C of a molding machine. By this constitution, a hollow molded product uniform in thickness and excellent in strength and appearance can be efficiently produced.