To obtain a synthetic resin foam which does not destroy the ozone layer and is excellent in compression strength and heat insulating properties by thermally melting an alkonylarom. resin, incorporating a blowing agent comprising a fluorohydrocarbon having no chlorine atom, a hydrocarbon, and an alkyl chloride into the resin melt, cooling the resultant gel to a temp. suitable for foaming, and subjecting the gel to extrusion foaming.

A styrene resin is pref. as the alkenylarom. resin. The blowing agent comprises 30 wt.% or higher but lower than 70 wt.% fluorohydrocarbon having no chlorine atom and 30 wt.% or higher but lower than 70 wt.% blowing agent comprising at least either a hydrocarbon or an alkyl chloride. The fluorohydrocarbon having no chlorine atom is selected from among difluoromethane, 1,1-difluoroethane, 1,1,1,2tetrafluoroethane, etc., in terms of thermal insulating performance and solubility in the resin. Propane, butane, etc., are pref. as the hydrocarbon in terms of stability and cost. Ethyl chloride is pref. as the alkyl chloride in terms of plasticizing effect, permability, and hygiene.