PURPOSE: To generate a desired focus voltage by providing a transformer and a transistor(TR) connected in series to the circuit, inputting a triangular wave voltage to the TR and generating a parabolic form voltage to a secondary winding of the transformer.

CONSTITUTION: A transformer 9 having a large stepup ratio and a leakage inductance is connected in series with a TR 8 driving the transformer 9. Then a waveform of a voltage inputted to a base 8b of the TR 8 and fed to a primary winding of the transformer 9 is formed to be an asymmetrical triangular wave. The current flowing to the transformer 9 depends on the leakage inductance in existence at the secondary winding 9b of the transformer 9 and an impedance resulting from converting a resistance of a resistor 11 with respect to the primary winding 9a and its current waveform is the integration of an input signal voltage 10. In this case, since a harmonic component of the voltage generated across the resistor 11 is lost by a stray capacitance of a focus electrode 2 connected in parallel, the waveform is parabolic. Thus a desired focus voltage is obtained by using the TR 8 having a low breakdown voltage and a DC power supply with a low voltage.