PURPOSE: To prevent the displacement, peeling wrinkling or occurrence of voids of a heat-shrinkable covering material that would occur when it is heated to be shrunk, by joining a heat-shrinkable auxiliary sheet whose rate of heat-shrinkage is low to a part of a main pipe around a neck section of a branch pipe.

CONSTITUTION: A heat-shrinkable auxiliary sheet 1 having an adhesive layer is placed to a main pipe 23 of a T-pipe 2 adjacent to a neck section 22 of a branch pipe 21, and is heated with a flame of a gas burner to be shrunk and thermally joined to the main pipe 2. The rate of heat-shrinkage of the auxiliary sheet is on the order of about 3W30% and preferably 5W25% and therefore the auxiliary sheet would not be displaced away from the neck section 22 of the T-pipe or would not have wrinkles when heated and shrunk. A first heat-shrinkable sheet 3 having an adhesive layer is placed on all the remaining circumference of the main pipe 23 with part thereof overlapped with the auxiliary sheet 1. Then a second heat-shrinkable sheet 5 having an adhesive layer is placed around the branch pipe 21 with the opposite ends overlapped and joined by a heat seal tape 6.