PURPOSE: To enable to mix uniformly a resin and a foaming agent so as to prevent the foaming agent from suddenly leaking, by forming a plug flow of a resin at a pressure higher than the injection pressure of a foaming agent at an extrusion side of a resin feeding part of a screw extruder, and injecting the foaming agent at the forward end of said flow.

CONSTITUTION: A resin stagnating part E without screw blades 4 is formed at the extrusion forward end of the melted resin feeding part A of the screw type extruder, and the plug flow of the resin having a resin pressure higher than the injection pressure of the foaming agent is formed at the resin stagnating part E. The foaming agent is injected at a foaming agent injection part F on the extrusion side of the plug flow, the resin and the foaming agent are compressed and mixed in the state of liquids at a compressing section B on the extrusion side, and the mixture is extruded through a metering section D from a discharge outlet 8.