To obtain a flame-retardant resin composition capable of exhibiting excellent flame retardance and workability by mixing a thermoplastic resin with a phosphorus-based compound and a specific resin as an auxiliary having heat resistance higher than that of a conventional resin.

This composition is obtained by mixing (A) a thermoplastic resin (preferably a polyolefin-based resin, a polystyrene-based resin or a polyphenylene ether-based resin) with (B) a hydroxybenzaldehyde/phenol resin (C) a phosphorus-based compound (preferably red phosphorus, a phosphoric ester, ammonium polyphosphate or melamine phosphate). Preferably 100 pts.wt. of the component A is mixed with 0.5-100 pts.wt. of the component B and 0.5-50 pts.wt. of the component C. Preferably the component A is mixed with a mixture or a molten kneaded material of the component B and the component C and melted and kneaded to give the composition. Preferably the composition is further mixed with 0-200 pts.wt. based on 100 pts.wt. of the component A of an inorganic hydrate.