to a constant voltage source which consists of resistor 17 and zener diode 13 and the base of another transistor 12b is connected to the resistor 11 which converts the emitter current of transistor 10 into a voltage therefore if the emitter current of transistor 10 decreases the base voltage of transistor 12b decreases the collector current of transistor 12b decreases and the collector current of transistor 12a increases if the collector current of transistor 12a increases the base voltage of transistor 15b decreases the collector voltage of control transistor 15b increases and the voltage at the high voltage output terminal 7 increases via the fly back transformer 5 so that the voltage drop at the terminal 7 is compensated according to the embodiment of the present invention employing the common emitter type transistor 15b as a control transistor if the emitter voltage denoted by 24 in fig2 of the control transistor 15b is set for example to 100 volts the base voltage denoted by 25 in fig2 becomes about 99 volts in the conventional circuit of fig1 if the collector voltage denoted by 19 in fig1 a of the control transistor 15 is set to 100 volts the emitter voltage denoted by.