Provided are a non-halogen insulating tube and heat shrinkable tube which can achieve mechanical properties, heat resistance, and flame retardancy, and which has a flame retardancy that can pass the vertical combustion test even when the inner diameter of the tube is made smaller. The insulating tube is formed by shaping a flame-retardant resin composition containing a thermoplastic resin, a polyfunctional monomer, and an organic phosphorus-based flame retardant into a tube. The thermoplastic resin contains 5 mass% or more, relative to the entire thermoplastic resin, of either a resin having an unsaturated carbon-carbon bond or a resin containing a carbonyl group. The organic phosphorus-based flame retardant is selected from one or more of the following: a metal phosphinate, a melamine phosphate compound, an ammonium phosphate compound, and/or a polyphosphazene compound obtained by means of ring-opening polymerization of a cyclophosphazene. The content of the organic phosphorus-based flame retardant is 5 to 100 parts by mass per 100 parts by pass of the thermoplastic resin and the content of the polyfunctional monomer is 1 to 20 parts by mass per 100 parts by mass of the thermoplastic resin.