PURPOSE: To stably supply main raw material and additive at a constantly held composition ratio and to easily switch foaming magnification by quantitatively supplying the additive together with the material direction to the raw material supply unit of an extruder.

CONSTITUTION: Main raw material such as, for example, thermoplastic resin is introduced from a main raw material introducing hopper 4 directly to the raw material inlet 3 of an extruder 1 via a main raw material supply feeder 5. Various additives from an additive introducing hopper 6 are quantitatively introduced directly to the inlet 3 via independent additive metering screws. The feeder 5 is inverter-controlled at the number of revolutions of a motor to introduce the predetermined quantity of the thermoplastic resin directly to the inlet 3. On the other hand, foaming agent is introduced, similarly to the main raw material by the additive metering screw in the adding amount to match to foaming magnification directly to the inlet 3. Since the respective materials are independently introduced directly to the inlet 3, even if the shapes and the bulk densities of the materials are different, no classification occurs, and the materials of predetermined composition ratio is accurately introduced.