PURPOSE: To rationalize the facility capacity and lessen driving ac power and thereby provide a coil power supply which has a high power factor by connecting a bypass coil in parallel with a power supply for bypassing part of coil current by means of the bypass coil and lessening current which flows in a converter.

CONSTITUTION: Current from a converter 1 of low voltage and large current is allowed to flow in a poloidal field coil 2. In parallel with a bypass coil 3, a converter 4 of high voltage and low current is connected and the poloidal field coil current is so controlled by the converter 4 that it may flow into the bypass coil 3 and never flow in the converter 4 of high voltage and low current. When the current of the poloidal field coil 2 sharply lessens, the converter 4 generates high voltage and thereby the current of the poloidal field coil 2 is lessened. At that time, the converter 1 of low voltage and large current does not generate voltage at all and the voltage generated by the converter 4 is applied to the poloidal field coil. By this method, the facility capacity can be rationalized and driving ac power can be lessened.