PURPOSE: To reduce aberration by reducing a supply quantity of a heating current per one time to more extent as FL times is smaller, on the basis of a signal indicating flushing(FL) times form a counting means for easing up an increase in leader voltage due to an increase in FL times.

CONSTITUTION: When acceleration voltage and leader voltage are impressed on an emitter 1 to continue field radiation of an electron beam, unevenness is generated in the tip periphery due to the impact of positive ions to destroy an emission pattern. Then, when an FL switch 13 turns ON, a signal for indicating a first time of FL is sent form a counting circuit 12 to a timer 11. The timer 11 controls an ON/OFF circuit 10 based on the information of a memorized heating current supply time to supply the heating current to a filament 2 for a prescribed time. Similarly, 2 to (n) times FL are performed, a supply time is slightly lengthened in company with times of FL. When the pulse width reaches the upper limit value, it is fixed.