The invention discloses a flame-retardant thermoplastic resin composition without containing any halogen compounds as a flame-retardant agent but still exhibiting excellent flame retardancy. The resin composition comprises: (a) a thermoplastic resin kneadable or moldable at a temperature lower than 250.degree. C. such as polyethylene; (b) magnesium hydroxide as the principal flame retardant agent; (c) an organopolysiloxane compound of a specified viscosity; and (d) a reactive catalytic compound which is a radical-generating compound having an incipient thermal decomposition temperature higher than 250.degree. C. such as 2,3-dimethyl-2,3-diphenylbutane. Alternatively, the catalytic compound as the component (d) can be a combination of a noble metal, e.g., platinum, compound and a vinyl group-containing organosiloxane compound.