A cathode mechanism of an electron gun includes a crystal to emit a thermal electron from an end surface by being heated, a holding part to hold the crystal in a state where the end surface is exposed and at least a part of other surfaces of the crystal is covered, a first supporting post and a second supporting post each to support the holding part and extend while maintaining an unchanged sectional size, a first base part to fix the first supporting post, and a second base part to fix the second supporting post, wherein the holding part, the first supporting post, the second supporting post, the first base part, and the second base part are formed in an integrated structure made of the same material, and the crystal is heated by supplying a current to the integrated structure.