PURPOSE: To simply detect the pressure drop of inert gases hermetically sealed in a vessel under high pressure, by utilizing a discharge phenomenon.

CONSTITUTION: Two needle-shaped electrodes for discharge 5a, 5b are oppositely positioned into a vessel, to which inert gases under high pressure is enclosed, at regular intervals, and discharge limit voltage obtained from relationship with reference gas pressure is beforehand applied between these electrodes. When gas pressure in the vessel drops, in insulation degree lowers, and discharge is generated between the needle-shaped electrodes for discharge. Thus, the output V0 of a detecting circuit 7 becomes to 0, and output V1 is formed in a comparator 8.