PURPOSE: To facilitate electric discharge even at an area with low pressure by providing a filament to the inside of a vacuum gage and making a thermion and gas generating from the filament a trigger of the electric discharge.

CONSTITUTION: The filament 9 is provided to the inside of an electrode through a hole made on the cathode 2 or an insulator 10 via the insulators 11 and 12 in a vacuum vessel 4. In case the electric discharge is not generated by closing a point of contact 13 and impressing the electrode with the high voltage by an electric source 7, a point of contact 15 is closed and electricity is turned on the filament 9 and the thermion and the gas adsorbed on the filament are discharged by which both make the trigger and generate the electric discharge and the pressure measurement is made possible. Once the electric discharge is generated within the measuring range of the vacuum gage, the subsequent measurement is made possible.