An injection molding method comprising: a mold clamping step of clamping a first mold and a second mold to form a mold cavity; a first injection filling step of injecting a foamable molten resin into the mold cavity to fill an interior of the mold cavity with the foamable molten resin, after completion of the mold clamping step; a mold cavity expansion step of expanding the mold cavity by a specified quantity to cause the foamable molten resin to foam, after the start of the first injection filling step; and a second injection filling step of pouring a molten resin or gas into the foamable molten resin within the mold cavity, after the completion of the first injection filling step, and after the start of the mold cavity expansion step.