To provide a non-crosslinked flame-retardant resin composition delivering excellent heat resistance without deteriorating over a long period of time, as well as possessing sufficient flame retardancy, mechanical properties, flexibility and workability, and an insulated wire and a wiring harness using the same. The non-crosslinked flame-retardant resin composition includes (B) a metallic hydrate, (C) a hindered phenolic antioxidant, (D) a sulfurous antioxidant, and (E) a metallic oxide in (A) a non-crosslinked base resin which includes a propylene resin containing 50 wt % or more of propylene monomer. It is preferable to utilize an imdazole compound as the ingredient (D) and an oxide of zinc as the ingredient (E). In addition, 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.