A flame-retardant resin composition comprising 100 parts by weight of a rubber-reinforced polystyrene resin, from 0.1 to 20 parts by weight of an epoxy-modified phenolic resin, and from 1 to 30 parts by weight of an aromatic phosphate has good flame retardancy and good lightfastness, still having good mechanical characteristics intrinsic to the thermoplastic resin therein.