A foam molding method, a foamed plastic formed by the method, and a foaming agent used in the foam molding method are provided. The foam molding method may include rotating a screw provided in a barrel, feeding a granular or powdered raw material and a foaming agent into the barrel, the foaming agent producing foam through a chemical reaction, melting the fed raw material and producing a gas from the foaming agent, and phase-changing the produced gas to a supercritical state and mixing the gas with the melted raw material.