PURPOSE: To obtain a foamed resin product of high extrusion rate which is fine and uniform in quality by designing each of those units such as the supply unit of a screw, compression unit, No.1 weighing unit, mixer and No.2 weighing unit to be of special construction respectively.

CONSTITUTION: Foam generative thermoplastic resin in a hopper 1 enters a cylin der 3 through the hopper's outlet 2. A screw 4 in the cylinder is equipped with the following units in sequence. In other words, they are a supply unit 5 wherein the clearance between the cylinder and the screw is gradually reduced in order, No.1 weighing unit 7 of which compression ratio is between 1 and 2.5 times, having a barrier flight 7a which separates liquid resin from solid resin and No.2 weighing unit of which groove is deeper than that of a groove for solid resin of the No.1 weighing unit 7 by 10W50%, which is equipped, at its top end, with a torpedo 9b of which length is 0.5W3 times the inner diameter of the cylinder, and of which clearance between the cylinder is 0.3W3mm. Resin is uniformly extruded by said units of the screw at a low temperature.