Provided is a flame-retardant thermoplastic resin composition in which any halogenated flame retardant is not used but a phosphoric flame retardant is used, which has flame retardancy meeting U.S. UL Standard 94 V-2, which exhibits excellent dripping performance during burning, and which is excellent in balance between properties. The flame-retardant thermoplastic resin composition contains 100 parts by mass of a copolymer mixture containing a graft copolymer (A) obtained by graft-polymerizing a rubbery polymer, a vinyl cyanide compound, and a copolymerizable vinyl compound and a vinyl copolymer (B) which is obtained by copolymerizing the vinyl cyanide compound and the copolymerizable vinyl compound and which has a mass-average molecular weight of 90,000 to 160,000 and 5 to 20 parts by mass of a phosphoric flame retardant (C) blended with the copolymer mixture. The content CB1 of vinyl cyanide units in the acetone-insoluble fraction of the copolymer mixture is 22.0% to 32.0% by mass, the content CB2 of vinyl cyanide units in the acetone-soluble fraction thereof is 22.0% to 34.0% by mass, and |CB1−CB2| is 0% to 3.0% by mass.