Disclosed is a styrene-based resin composition that comprises 5-20 parts by mass of (D) a fire retardant per 100 parts by mass of a resin composition (1) that comprises 30-51 mass % of (A) a graft copolymer obtained by graft polymerization of a diene rubber polymer with an aromatic vinyl monomer and an unsaturated nitrile monomer, 10-55 mass % of (B) a copolymer comprising two or more monomers selected from among aromatic vinyl monomers, unsaturated nitrile monomers and unsaturated carboxylic acid alkyl ester monomers, and 15 to 39% by mass of a copolymer (C) comprising one kind of unsaturated carboxylic acid alkyl ester monomers, or two or more unsaturated carboxylic acid alkyl ester monomers, ((A)+(B)+(C)=100 mass %), wherein the diene rubber polymer constitutes 15-25 mass % of the resin composition (1).