TECHNOLOGY FOCUS - POLYMERS - Preferred Properties: The air bubble which forms the foam is mainly comprised from air bubble of 0.25 mm or less of diameter, and air bubble of 0.3-1 mm of diameter. The air bubble of 0.25 mm or less of diameter has 2-90% per foam cross-section of occupied area rate. The styrene resin foam has density of less than 50 kg/m3.Preferred Composition: The compound(s) chosen from saturated hydrocarbon with carbon number of 3-5 is contained in portion or all of foaming agent. The styrene resin foam further contains phosphorus-containing compound as flame retardant. 0.1-100 wt. pts. (weight parts) of ammonium sulfate is contained with respect to 100 wt. pts. of styrene resin. The styrene resin foam contains 100-1 wt.% of one or more types of compound chosen from saturated hydrocarbon with carbon number of 3-5, and 0-99 wt.% of another foaming agent with respect to foaming agent whole quantity. The foam contains 1-10 wt. pts. of compound(s) chosen from saturated hydrocarbon, with respect to 100 wt. pts. of foam.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Compounds: The saturated hydrocarbon with carbon number of 3-5 is propane, n-butane or i-butane. The other foaming agent is one or more types of compound chosen from dimethyl ether, diethyl ether, methyl-ethyl ether, water and carbon dioxide. The phosphorus-containing compound contains one or more types of compound chosen from red phosphorus, aromatic phosphoric ester and phosphorus-nitrogen containing compound. The flame retardant is one or more types of compound chosen from nitrogen compounds other than ammonium sulfate and boron-containing compound. EXAMPLE - Polystyrene resin G9401 (in wt. pts.) (100) and ammonium sulfate (10) were blended. Subsequently, a dry blend of resin mixture consisting of talc (0.5) and barium stearate (0.25), was carried out. The resulting resin mixture was supplied to an extruder of 65 mm of aperture diameter. The resin mixture supplied to the extruder was heated at 200 degrees C, melted or plasticized and kneaded. The resin was cooled to 120 degrees C, and a rectangular parallelepiped-like extrusion foaming material was obtained. Subsequently, a foaming agent consisting of 100% propane (4) was introduced into the resin from front-end vicinity of the extruder, with respect to polystyrene resin (100), and styrene resin foam was obtained. The foam had density of 34 kg/m3. The foam had excellent flame retardance, stability and thermal insulation property.