A flexible tube is placed in large and small furnaces separated by a heat-shield panel, and two kinds of jacket resin of the flexible tube are heated separately at different temperatures for their own softening points. A connecting area of the jacket resin enters the small furnace to a predetermined distance. In this connecting area, the jacket resin fails to reach the softening point temperature, and is not firmly welded to a blade. A cylindrical attachment is used to surround the spot that has lower temperature than the softening point, and hot air of approximately 250° C. is supplied from a spot heater into the cylindrical attachment. The spot in the cylindrical attachment reaches the softening point in a short time, and the jacket resin is welded to the blade.