PURPOSE: To obtain a vacuum meter capable of being used in an ultra-high vacuum apparatus to be cooled to a liquid helium temp. or an ultra-high vacuum apparatus made of an aluminum alloy, by utilizing ultraviolet rays as the ionization source of residual gas.

CONSTITUTION: A vacuum tank 2 is irradiated with ultraviolet rays through a window 3 from a laser beam source 1 with a wavelength of 2,000W4,000 in an ultraviolet region. The irradiated ultraviolet laser beam is again passed through the window 3 by the mirror 4 in the vacuum tank to be incident to an external monitor 5 where the oscillation state of the ultraviolet laser beam source is monitored. The residual gas in the vacuum tank is irradiated with ultraviolet laser beam to be ionized while the ionized ion is detected by a detector 8 while applying an electric field to electrodes 6, 7. The electrodes 6, 7 each can be formed into a configuration such as a cylindrical electrode or a flat plate electrode. One of the electrodes 6, 7 can be connected to a metal vacuum tank made of stainless steel or an aluminum alloy and the other electrode can be connected to the external terminal to be connected to the terminal of the detector 8.