To provide a flame-retardant thermoplastic resin composition which is free from the deterioration of physical properties even when a large amount of an inorganic flame retardant is incorporated, is excellent in heat resistance and low temperature resistance and also is excellent in the processability for insulation displacement contact.

The flame-retardant thermoplastic resin composition is prepared by melting/kneading (A) 100 pts.mass of a thermoplastic resin component, (B) 120-300 pts.mass of an inorganic flame retardant, (C) 0.001-2 pts.mass of an organic peroxide, (D) 0-4 pts.mass of a (meth)acrylate-based and/or allyl-based crosslinking aid and (E) 0-2 mass% of a silane coupling agent based on the amount of the component (B), at a temperature of the 1-minute half-life temperature of the organic peroxide (C) or higher. The above thermoplastic resin component (A) comprises (a) 10-80 mass% of a propylene-based elastomer and/or propylene-based plastomer, (b) 10-80 mass% of a crystalline propylene-based resin and (c) 5-25 mass% of other components.