PURPOSE: To make possible necessary operations such as opening and closing operations of valves of exhaust sequence and controlling of applied voltage of an electron gun by providing a tertiary coil for generation of low voltage to a transformer for ion pump power source.

CONSTITUTION: The primary coil 2 of a step-up transformer is energized by AC input and the high voltage output of its secondary coil 3 is supplied to an ion pump 12 through a full-wave rectifying circuit, thereby driving the pump 12 and maintaining the inside of the lens barrel of a scanning type electron microscope at a high degree of vacuum. The voltage l0 generated by the tertiary coil 4 for generating low voltage is supplied via half-wave rectifying circuit to the base of a switching transistor TR 18, by the conduction of which an LED 19 is let to emit light, indicating the conduction state. In addition, if the TR 18 becomes non-conducting, an output voltage appears in terminal 25, whereby it is detected that the driving of the ion pump 12 has been stopped.