PURPOSE: To reduce the influence of ringing at the time of a high frequency operation and to obtain high performance.

CONSTITUTION: An auxiliary wiring 6 for obtaining the pulse of a scan period, a rectifying diode 8 rectifying a voltage generated in an auxiliary wiring 6 and a claming capacitor 10 are provided on the secondary side of a flyback transformer. Thus, clamping voltage is generated and the primary side voltage of the flyback transformer is composed by the voltage. When the primary side voltage of the flyback transformer is clamped, the influence of ringing in a high voltage output voltage is reduced since the influence of ringing on account of leakage inductance and distribution capacitance of the transformer is impervious to reception. Then, high voltage regulation performance which is satisfactory in the high frequency operation is obtained.