PURPOSE: To make it possible to create a third power supply voltage without requiring any dedicated power supply circuit.

CONSTITUTION: The power supply circuit comprises a first capacitor C1 for storing the voltage difference of a first DC power supply ±12V as a first DC voltage, a second capacitor C2 for storing a second DC voltage obtained by adding the first DC power supply voltage to the first DC voltage, a third capacitor C3 for storing a third DC voltage(116V) obtained by adding 80V DC voltage of a second DC power supply to the second DC voltage, and a fourth capacitor C4 for reversing the polarity of the third DC voltage, wherein a third voltage higher than the sum of voltages of first and second low DC power supplies is employed as a grid bias voltage VG1.