PURPOSE: To only measure an ion current value periodically varying, and heighten a precision of measurement, by periodically interrupting an electron current in a square wave pattern, and neutralizing a change of each electrode voltage not to affect the collector electrode with a leak current made to DC current.

CONSTITUTION: With a square wave oscillator 22 generating a square wave, during that period, to the cathode is applied a positive voltage of a power unit 23 and to a first grid 12 applied a negative voltage of a power unit 24, with an electron current cut off. And between the cathode 13 and the first grid 12, are connected a resistor 16 and 17, with its center point connected to a collector 10, and a voltage change by the square wave oscillator 22 is made not affecting the collector 10. A changing component, periodically generated in a current flowing in the collectot 10, is amplified by an AC amplifier 21, and measured by an indicator 20, and only an ion current value is separated from the leak current and obtained. In this way, a vacuum can be measured in high precision.