PURPOSE: To keep the welding strength constant and to certainly prevent defects including separation of the welded point by a method wherein the temperature of heating applied to the welding to be made on overlapped parts on both ends of a hoop made of thermoplastic synthetic resin for binding is adjusted so that it can be maintained at a specified temperature that is above the softening temperature of the hoop.

CONSTITUTION: Both ends of a hoop 2, wound round a coil 1, are overlapped each other through a metal 3 and are subjected to high frequency wave, and thereby high frequency induction heating is applied to the metal 3. Although the temperature of said heating is adjusted by voltage of the applied high frequency wave, the heating temperature must be above the softening temperature of thermoplastic synthetic resin, the material of the hoop 2. Taking said softening temperature as Ts, the heating temperature must be, in order to obtain a maximum welding strength, within a range of Ts+0.05Ts-Ts+0.225Ts. In the case where the hoop 2 is made of polyester having the softening temperature of 400°C, the heating temperature will be in the range of 420-490°C. After both the ends of the hoop 2 are heated and softened in the way as mentioned above, the metal 3 is removed and both of the ends are welded together.