Abstract: A fire-retardant thermoplastic resin composition consisting essentially of:(A) a thermoplastic synthetic resin,(B) about 40 to 150 parts by weight, preferably about 70 to 120 parts by weight, per 100 parts by weight of the thermoplastic synthetic resin, of at least one magnesium-containing inorganic compound selected from the group consisting of magnesium hydroxide, basic magnesium carbonate hydrate and hydrotalicites, and(C) a fire-retardant assistant selected from the group consisting of (1) about 0.1 to 30 parts by weight, per 100 parts by weight of the magnesium-containing inorganic compound, of an alkali metal chloride, (2) about 0.2 to 5% by weight, calculated as metal per 100 parts by weight of the magnesium-containing inorganic compound, of an inorganic tin compound, and (3) a combination of the alkali metal chloride (1) with about 0.