To provide a practically effective flame-retardant thermoplastic resin composition which is excellent in flame retardancy, electrical safety and mechanical properties, hardly generates gas in molding process, and substantially avoids bleed-out even at high temperature and high humidity.

The flame-retardant thermoplastic resin composition contains 100 pts.wt. of a thermoplastic resin and 5-50 pts.wt. of a polymer of an alkenyl phosphorus compound represented by formula (1), wherein R1, R2 and R3 represent H or a 1-20C monovalent hydrocarbon group; R4 and R5 represent H or a 1-18C monovalent hydrocarbon group; and m and n represent 0 or 1.