PURPOSE: To obtain composite material of thermosetting resin/foam excellent in weight-saving, mechano-physical properties, heat insulating properties and sound barrier properties by a method wherein the composite material is made of polyvinylidene chloride-based multi-cellular expanding particle, the particle diameter and expansion ratio of which are specified, and thermosetting resin.

CONSTITUTION: The composite material concerned is made of 10-99vol.% of polyvinylidene chloride-based multi-cellular expanding particle having the particle diameter of 0.01-5mm and the expansion ratio of 300-10,000% and 1-99vol.% of thermosetting resin. Further, foaming agent-containing polyvinylidene chloride- based solid beads having the particle diameter of 0.005-2mm and thermosetting resin are mixed with each other under the mixing ratio in weight of bead/ thermosetting resin stock of 100/1-1/30 so as to simultaneously set and expand by reaction heat at the setting of the thermosetting resin and by said reaction heat and heat supplied from outside.