To provide an ionization gauge that outgases a small amount of gas and has a robust cathode.

A hot cathode in a hot cathode ionization gauge (for example: hot cathode B-A type ionization gauge) is replaced by a cold cathode by a carbon nanotube. The drawing shows a vessel 1, a cathode 5 where the carbon nanotube is fixed to the inner surface of a metal cylinder, a drawing electrode 4 for promoting electron emission, a collecting electrode 3, an ion collecting electrode 2, and an introduction electrode 6.