PURPOSE: To prevent the dew condensed water by disposing a spiral metallic wire on the inner surface of a resin pipe formed of thermoplastic resin, and heating this spiral metallic wire by current application to provide a spiral groove at the contact part of the resin pipe inner surface and to fit the metallic wire into this spiral groove.

CONSTITUTION: A jig for contracting the diameter of a spiral metallic wire 21 is set to the spiral metallic wire 21, and the spiral metallic wire 21 is inserted along with the jig into a polyethylene resin pipe 11 with the smooth inner surface. The jig is then removed from the spiral metallic wire so as to eliminate diameter contracting force, and the spiral metallic wire 21 is made to press contact with the inner surface of the resin pipe 11. Electrodes are connected to both ends of the metallic wire 21, and a current is applied to heat for the specified time. The inner surface of the resin pipe 21 is softened or fused by this heat, and a recess is formed at the inner surface of the resin pipe 11 by the pressure caused by the restoring force of the metallic wire 21 so as to form a spiral groove 12. The metallic wire 21 is fitted into this spiral groove 12 and integrally fixed to the resin pipe 11. Strength increase action by the spiral metallic wire can therefore be displayed sufficiently.