To provide a non-crosslinked flame-retardant resin composition possessing sufficient flame retardancy, mechanical properties, flexibility and workability, and also delivering excellent heat resistance over a long period of time as it is hard to be molten when used under high temperature environment and its material is not deteriorated even if used in contact with a vinyl chloride resin material and the like, and an insulated wire and a wiring harness using the same. The composition includes (A) a non-crosslinked base resin which contains a propylene resin containing 50 wt % or more of propylene monomer and a thermoplastic resin of which a melting point is 180.degree. C. or more, (B) a metallic hydrate, (C) a hindered phenolic antioxidant, (D) a sulfurous antioxidant, and (E) a metallic oxide. Polymethylpentene, an imidazole compound and an oxide of zinc are preferably utilized as the thermoplastic resin, the ingredients (D) and (E), respectively. The composition is used as an insulated covering material for a non-halogenous insulated wire, which is used in a wire bundle of the wiring harness.