PURPOSE: To improve the efficiency of a magnet and measuring accuracy by placing the magnet in a vacuum container as cathode to simplify the structure while allowing the formation of a nude form.

CONSTITUTION: A vacuum gauge is made up from a power source 21 in which a magnet 11 serving as discharge cathode concurrently is provided inside a vacuum container 30 to generate an electric field having a component orthogonal to a line of magnetic force generated by the magnet 11 and a measuring device 22 to measure a discharge current. The magnet 11 can be inserted into or taken out of the container through a very narrow parts as the diameter of thereof can be adjusted larger or smaller up to about 15mm as required. There is no need for making an anode 40 separately because the container 30 plays the role thereof and the anode can be set only if a gap is provided slightly larger than about 15mm in substance. Because of the absence of the anode 40, it is suffice that the magnet is inserted into the vacuum container to supply power, thereby simplifying the construction. This also can reduce the weight of the magnet 11 to less than one-fifth, thereby improving the efficiency as well.