TECHNOLOGY FOCUS - POLYMERS - Preferred Foam: The air bubbles forming the foam comprises air bubbles of diameter 0.25 mm or less and air bubbles of diameter 0.3-1 mm. The air bubbles of diameter 0.25 mm or less occupies 2-90% of the foam cross-section.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Foaming Agent: The non-halogen foaming agent is chosen from 3-5C saturated hydrocarbon. The foaming agent contains 100-1 wt.% of 3-5C saturated hydrocarbon and 0-99 wt.% of another foaming agent. 1-10 weight parts (wt.pts) of compound chosen from 3-5C saturated hydrocarbon is contained as non-halogen foaming agent with respect to whole quantity of foaming agent in 100 wt.pts of the foam. The other foaming agent is chosen from dimethyl ether, diethyl ether, methyl ethyl ether and carbon dioxide.Preferred Flame Retardant: The flame retardant is red phosphorus coated with thermosetting resin, metal hydroxide, and/or metal. 0.1-15 wt.pts of stabilized red phosphorus is contained with respect to 100 wt.pts of styrene resin. phosphorus containing compound other than red phosphorus, nitrogen compound and/or boron containing compound are further contained as flame retardant.Preferred Foam: The foam corresponds to combustible specification of extrusion molded polystyrene foam heat insulating board specified by JIS A9511. The foam corresponds to two sorts of extrusion molded polystyrene foam heat insulating board or three sorts of insulative specification specified by JIS A9511.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Hydrocarbon: The 3-5C saturated hydrocarbon is propane, n-butane or i-butane. EXAMPLE - Polystyrene resin (in wt.pts) (100) and stabilized red phosphorus (10) were mixed. Talc (0.5) and barium stearate (0.25) were dry blended with resin mixture. The obtained resin mixture was supplied into an extruder, heated for melting, plasticized and kneaded. The mixture was cooled to 120degreesC and a rectangular extrusion foam was produced. A foaming agent consisting of propane was pressed into an extruder during the process. The foam had density of 31 kg/m3, favorable heat conductivity and favorable combustibility when measured based on JIS A9511.