PURPOSE: To enable the measurement upto a hollow vacuum region by stabilizing a potential gradient on the surface of a magnetron type discharge electrode at a steep part thereof through control of possible discharge with a control electrode attached theeto along the length of a hollow section piercing an anode in such a manner that the anode and a cathode are electrically insulated.

CONSTITUTION: After the magnetic field reaches a fixed intensity, a controller 23 gives a starting signal to a control power source 26 to output a high voltage Va. A specified positive high voltage Va is applied to an anode 1 to generate a crossed field discharge in a space surrounded by two plate-shape sections 3a and 3d and a bar-shaped section 3c of the anode 1 and a cathode 3, allowing current to flow through the anode 1, the cathode 3 and a control electrode 6 separately. A measurement indication circuit 22 measures the low frequency component 1g of the current during the discharging of the control electrode 9 and indicate the value as vacuum.