PURPOSE: To enable to perform a wide-ranged measurement, by a method wherein an ion current, generated resulting from a discharge, is measured through the installation of a hollow cylinder anode, a cathode covering the opening end of the anode, a control electrode on the axis of the hollow part, and a coil emanating a magnetic line parallel to the axis.

CONSTITUTION: A hollow cylindrical anode 1, cathodes 3 convering the opposite ends of the anode, a linear control elctrode 5 in the proximity of an axis of a hollow part 2, and checking electrodes 8 surrounding said electrodes and provided with a vent hole are mounted in a vacuum vessel 14, and the electrodes are connected to a power source through conductive wires 12aW12c. An air-core coil is placed outside the vacuum vessel 14 to form a magnetic field in a direction of an axis. An anode voltage Va, a control electrode Vg, and a cathode voltage Vk are applied at the relation of Vk<Vg<Va<0. A check electrode 8 is earthed. An electric field, produced by a electric potential between the electrodes, and said magnetic field generate a crossed field discharge to produce an ion, and an ion current Ia is measured by a current measurement circuit 15 wherein much of the ion collects at the anode 1 to find the degree of vacuum by means of an indicator 16 for the degree of vacuum. This permits to perform a wide-ranged measurement which eliminates the need for a cooling device.