PURPOSE: To obtain a measuring element normally operated under the ferromagnetic field of a nuclear fusion reactor, receiving the reduced effect of a secondary electron and capable of measuring the rapid change in gas pressure, by arranging the electrodes of the measuring element in a concentric circular form.

CONSTITUTION: A needle electrode 3 is attached to the center of concentric electrodes and a reticulated filament 1 is attached outside a grid 2 in a concentric form. Positive potential is applied to the grid 2 and negative potential to the anode 3. The thermoelectron emitted from the filament 1 is accelerated by the positive potential of the grid 2 and ionizes a gaseous molecule in the space between the filament 1 and the grid 2. The ionized ion passes through the mesh of the grid 2 and is accelerated by the negative potential of the anode 3 and collected by the anode 3 to be converted to an ion current. By this mechanism, the area of the anode 3' can be reduced to a large extent and concentric electrode arrangement can be held and, therefore, a measuring element can be normally operated even under a ferromagnetic field of which the direction changes timewise.