PURPOSE: To miniaturize a transformer as well as to reduce an inverter circuit current by inputting those of three-phase voltage doubler rectification, single- phase full wave rectification and a single-phase half-wave rectification into an inverter according to a size of load.

CONSTITUTION: When a tube current is large enough, one of the voltage or current is inputted into a voltage control circuit 20 and a load condition judging circuit 22. In this case, this circuit 22 is outputted to three gate circuits 23-25 when the judgment is large, thereby controlling a silicon controlled rectifier, while it informs the effect of triple voltage rectification. The voltage control circuit 20 emits a control signal to an inverter 6 by a tube voltage setting signal and a signal out of the load condition judging circuit 22, setting a tube voltage to the setting value. This control signal is amplified in a base drive circuit 21 and fed to four transistors 7-10 of the inverter 6. When load is small, in response to this fact, single-phase double voltage rectified voltage, single-phase full-wave rectified voltage, single-phase half-wave rectified voltage or the like of the a three-phase 200v AC power source are used. According to this method, a transformer can be miniaturized, thus an inverter circuit current is reducible in addition.