PURPOSE: To obtain a resin composition which can give a molding that does not suffer from bleeding to the surface and generates neithercorrosive nor noxious gases during processing or in burning and which is markedly improved in flame retardancy and impact resistance without detriment to the features of the polystyrene resin.

CONSTITUTION: This resin composition is prepared by mixing 100 pts.wt. polystyrene resin with 5-80 pts.wt. product of melt kneading of a product obtained by modifying with a carboxylic acid anhydride a hydrogenated block copolymer obtained by hydrogenating a block copolymer composed of at least one polymer block based on a vinyl-aromatic compound and at least one polymer block based on a conjugated diene compound and at least one nonhalogenous flame- retarding compound selected from among a phenolic resin and phosphorus, nitrogen and boron compounds, and melt-kneading the resultant mixture.