To perform pull-back work of an insulating cover smoothly by applying a slip material to a part on the inner circumferential surface of an outer tube where an inner tube is not fixed thereby improving slippage.

An enlarged hole holding member is fixed to the inner circumferential surface of inner tubes 30, 31 and an outer tube 10 is enlarged at the opposite end parts thereof. A tube is inserted into the inner circumferential surface part 10A of the outer tube 10 where the inner tubes 30, 31 are not fixed and a slip material 50 is supplied to the tube and sprayed. The slip material 50 can impart slippage between the inner circumferential surface part of the outer tube and the sheath of the cable and has such properties as electric insulation and nonabsorbency of moisture. Since slippage is improved between the inner circumferential surface part of the outer tube and the sheath of the cable, pull-back work of an insulating cover can be carried out smoothly.