PURPOSE: To improve the machining efficiency without causing the excess heating of a machining part by putting a transformer tap at a strong side in a large size steel wire, putting the transformer tap at the strong side in a time of starting machining in a fine size steel wire and switching the tap to a weak side just before completing the machining.

CONSTITUTION: Pressing an end of a steel wire is started in a low pressure, a magnetic switch CR7 is put on at the same time when a timer TM3 to set the starting time of energizing to a transformer is made time-up, the current of a strong side is energized to the transformer TR and heating of the top end part of the steel wire is started with the large current value. When the setting time of a set timer TM52 is passed, the magnetic switch CR7 of the strong side is put off, at the same time, the magnetic switch CR4 of the weak side is put on, the current of the weak side is energized to the transfomer TR, it is heated with a small current value, heating is continued until the limit switch LS is put on as it is.