To provide a flame retardant thermoplastic resin composition having an excellent surface appearance, impact resistance and still further a low specific density.

The flame retardant thermoplastic resin composition comprises following (A)-(D) components, wherein the specific gravity is 1.16 or less and the UL flame retardance standard of V-0. The component (A) is 100 parts by weight of a rubber reinforced thermoplastic resin, which is prepared by polymerizing a monomer component consisting of an aromatic vinyl compound, or an aromatic vinyl compound and another vinyl compound copolymerizable with the aromatic compound in the presence of a rubber-base polymer, or a rubber reinforced thermoplastic resin comprising a mixture of the above rubber reinforced copolymer resin and a (co)polymer of vinyl monomers. The component (B) is 18-25 parts by weight of a bromine base flame-retardant. The component (C) is 1-6 parts by weight of an antimony compound. The component (D) is 0.01-10 parts by weight of at least one kind of a dispersing agent selected from a polymer containing a functional group, a modified silicone and a composite material of a silicone and silicate dioxide.