PURPOSE: To obtain the subject foam capable of being processed without being accompanied by the danger of ignition, not destruction an ozone layer and useful as a packing material by injecting a plurality of different foaming agents into a thermoplastic synthetic substance in a specific ratio.

CONSTITUTION: When a compsn. material is plasticized. mixed and homogenized in an extrusion molding apparatus 1 and two different foaming agents are injected into a thermoplastic synthetic substance through injection opening parts 6, 7 arranged before and behind on a downstream side, gaseous nitrogen (N2) is injected in an amt. of 0.2-0.5% by wt. of polystyrene mixed with talc as the first foaming agent and a liquid partially halogenated compd. (e.g.; CHCl2 F) is injected in an amt. of 0.5-2.0% on the basis of polystyrene mixed with talc as the second foaming agent and mixing pressure of at least 55 bar and mixing temp. of at least 130°C are adjusted in an extrusion molding head to obtain the objective foam.