To obtain a production process whereby a high-quality foamable resin compsn. is produced by extrusion without causing the premature foaming or the coagulation of a blowing agent, i.e., the cause of very large cells occurring at the step of foaming, even when a solid blowing agent, etc., is supplied into a melted thermoplastic resin at the intermediate point of an extruder.

A mixture of a thermally decomposable solid blowing agent and a thermoplastic resin is supplied into a thermoplastic resin, which has been melt-kneaded in an extruder, at an intermediate point of the extruder by using a feeder which can mix different kinds of materials.