A flame-retardant thermoplastic resin composition comprising (A) 100 weight parts of a thermoplastic resin, (B) 10 to 300 weight parts of particulate metal hydroxide; (C) 0.01 to 50 weight parts of a branched polyorganosiloxane having alkoxy groups and described by average unit formula R1a(R2O)bSiO(4−a−b)/2, where R1 and R2 are monovalent hydrocarbon groups selected from the group consisting of alkyl, alkenyl, and aryl groups, a is 0 or a positive number; b is a positive number; and a+b is a number from 0.75 to 2.5; (D) 0.01 to 50 weight parts of a branched polyorganosiloxane having silanol groups and described by average unit formula R3a(HO)bSiO(4−a−b)/2, where R3 is a monovalent hydrocarbon group selected from the group consisting of alkyl, alkenyl, and aryl groups, a is 0 or a positive number, b is a positive number, and a+b is a number from 0.75 to 2.5; and (E) 0.01 to 10 weight parts of a condensation reaction promoting catalyst. The present invention further relates to a method for manufacturing the flame-retardant thermoplastic resin composition.