PURPOSE: To provide a vacuum gauge capable of utilizing an ultra-high vacuum or extreme-high vacuum and having simple structure.

CONSTITUTION: A needlelike cathode 3 having a tip curvature radius of 1μm or less and an anode 2 opposite to the needlelike cathode 3, are provided in an electrode chamber 1a connected to a vacuum chamber 1 in an ultra-high vacuum or extreme-high vacuum, and the + side of an outside constant-voltage power supply is connected to the anode 2 and the -side is linked to the needlelike cathode 3 via an ammeter 5. The radiation current of the needlelike cathode 3 detected by the ammeter 5 is differentiated with a differentiation circuit 6, and then displayed on a display 7.