An injection-foaming method for a thermoplastic resin comprising using an injection unit 30 provided with a hopper 35, a plasticizing cylinder 31 and a screw 32, and a mold 10 having a mold cavity 10a of changeable capacity; filling the mold cavity 10a with a foaming-agent-containing melted resin injected from the injection unit 30; and then enlarging the mold cavity 10a for allowing the foaming-agent-containing melted resin to expand, wherein inclusion of a foaming agent into a thermoplastic resin is done by using a mixture of a foam nucleating agent and a foaming gas as foaming agent, feeding the mixture to hopper 35 or plasticizing cylinder 31 and screw 32 of injection unit 30 at a pressure of 0.1 MPa or more and less than 1.0 MPa, and contacting the mixture with a pre- or post-plasticized thermoplastic resin present in hopper 35 or plasticizing cylinder 31 and screw 32.