PURPOSE: To join the sides of a cover that is free from any dent and covers the surface of an insulating material with which a pipe is covered, by cutting a resin pipe having a diameter larger than the outer diameter of the insulating material to open the resin pipe, and covering the insulating material with this opened resin material.

CONSTITUTION: A thin-walled thermoplastic resin pipe 1 that is produced by extrusion is cut into a length (l). The diameter of the pipe 1 should have a diameter larger than the diameter of an insulating material 3 such that if the insulating material 3 with which a metal pipe 4 or the like is covered were covered with the pipe 1, overlapped sections 4 would be formed along the cut line 2. Then the protective cover 1' that has been produced by cutting the thermoplastic resin pipe 1 longitudinally along the cut line 2 is fitted on the insulating material 3 with the overlapped sections 3 formed. Holes are drilled by a drill in the overlapped sections 3 with a suitable interval between the holes, and blind rivets are driven in the holes so that the protective cover 1' is secured onto the insulating material 3.