PURPOSE: To make the whole power unit small-sized by feeding to an induction heating coil the generated electric power of medium frequency or high frequency by the rotary type frequency converting device equipping a fly wheel and variable speed driving device.

CONSTITUTION: Mutual butt face 9a is welded with its pressing by moving the right and left clamps 19 when the temp. of the butt face 9a is raised by electrifying an induction heating coil 18. The rotary type frequency converting device 1 as the power source device during resting time has no load in welding. The speed of the induction motor 2 equipping with a fly wheel 3 is increased upto the rated number of revolutions by a variable speed driving device 16 for this prescribed no load time. In case of the welding, the electric power is fed to an induction heating coil 18 via a matching device 17 from a generator 4 by connecting a clutch 5. The numbers of revolutions of an induction motor 2, generator 4 and fly wheel 2 are reduced by the power feeding to this load, but the output frequency is kept within the range of more than 50% of the rated value by the fly wheel effect of the fly wheel 3.