PURPOSE: To prevent a stain in an ionizing chamber and attain pressure measurement or mass spectrometry with high accuracy in the ultra-high vacuum atmosphere.

CONSTITUTION: An ionizing chamber 2 is made of a noble metal material which is inactive in adsorption of a gaseous molecule. A cold cathode incapable of generating an evaporation material is used as an electron source 6. In the case of use of a hot cathode, a shielding plate is disposed along an electron orbit 3, and only an electron is introduced into the ionizing chamber 2 via a reflection plate. Consequently, an ESD ion is not taken into a collector 5 so that a pressure or mass in the ultra-high vacuum atmosphere can be measured with high accuracy.