To provide an electron gun with improved current utilization efficiency.

In this electron gun, the side surface of a cathode 1 is surrounded by a substance 2 whose emission current density is low. The cathode 1 is made of Ta, whose peripheral substance is made of W. In this electron gun, few electron beams are emitted from the peripheral substance 2 of the cathode. No electron beam is emitted from the side surface of the cathode 1, so that the emission of the electron beams 4 decreases and moves to an optical axis together to form a pencil of lines.