TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: Thermoplastic resin (in weight%) (100) contains styrene resin (99-50) and phenol-type resin (1-50). The amount of sulfonyl-group containing compound in the resin foam with respect to resin (A) (100), is 0.1-50 pts.wt.. The amount of phosphorus-containing compound, nitrogen compound, boron-containing compound and/or polyhydric alcohol in the resin foam with respect to resin (A) (100), is 0.1-200 pts.wt..Preferred Compound: Phenol-type resin is novolak-type cresol-formaldehyde resin.Preferred Property: The combustibility of the thermoplastic resin foam is calculated based on JIS A 9511.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Compounds: Sulfonyl-group containing compound is aromatic sulfonamide-type compound and/or aromatic sulfonic acid-type compound. Aromatic sulfonamide type-compound is p-toluenesulfonamide and/or o-toluenesulfonamide. Aromatic sulfonic acid-type compound is benzene-sulfonic acid, p-toluenesulfonic acid, naphthalenesulfonic acid, their alkali metal salts and/or their derivatives. The foaming agent is non-halogen type foaming agent. Portion(s) of the foaming agent has 3-5C saturated hydrocarbon, and ether. EXAMPLE - StyronG9401 (TM) (polystyrene) (in pts.wt.) (85), novolak-type o-cresol-formaldehyde resin (15), Topsizer1-S (TM) (p-toluenesulfonamide) (5), titanium hydroxide-coated red phosphorus-polystyrene masterbatch (5) having average particle diameter of 5 micron, ignition point of 300 degrees C or more, and phosphine generation amount of 0.1 mug/g or less, TERRAJU C-60 (TM) (ammonium polyphosphate) (5), MC440 (TM) (melamine cyanurate) (5), isobutane (3) and dimethyl ether (4), were mixed, and melt-kneaded at 200 degrees C, to obtain pellet-form resin composition. The resin composition was heated, melt-kneaded and cooled, to obtain rectangular parallelepiped-like extrusion-foamed component (A). Component (A) had foam density of 40 kg/m3, and excellent flame retardance and form retentivity.