PURPOSE: To measure pressure without correcting the direction and intensity of a magnetic field by using a spherical electrode structure and gathering ions to a collector without reference to whether a magnetic field is present or not and its intensity.

CONSTITUTION: The same effect is obtained in any direction of the magnetic field by making the electrode spherical, and an equal ion current is supplied in the presence of the magnetic field having the same intensity. Further, even if the radius of rotation when ions are in spiral motion about the direction of the magnetic field as a ceonter axis varies, ions produced between a lead-out electrode 6 and the collector 7 are all gathered to the collector 7 with a magnetic field produced between two electrodes (6 and 7). Thus, the constant ion current is supplied under the same pressure regardless of the intensity of the magnetic field and its direction. Thus, a pressure measurement is taken without any correction.