To prevent a crack owing to cooling by executing induction heating through temperature controlling of welded region directly after welding to a temperature lower than the melting temperature using a high frequency heating coil.

A welding torch 1 is for executing MAG welding in a gaseous atmosphere of carbonic acid with addition of argon and is held at a tilt angle of 45° to a vertical direction. A high frequency heating coil 2 is held by making its center of hole coincident with the axis line P. The welding is executed on the welding objective range e comprising an outer peripheral surface 31 of a cylindrical body 3 of an assembling body 5a and an end surface 421 of a cylindrical body 4 following one round path in the peripheral direction. The welding objective region e is subjected to induction heating with the high frequency heating coil 2 simultaneously with the starting of welding by an arc A onto a wire 10. Simultaneously with the finish of welding, the high frequency heating coil 2 continues its operation for about 10 seconds and then stops with its output of heating parameter and frequency controlled to prescribed lower conditions. The welded region follows a gentle cooling which is held for a while at a temperature on the half way from the welding temperature and thus crack is prevented.