An ion gun including an anode, a cathode opposed to the anode and having a first portion and a second portion, and a magnet configured to form a magnetic field space between the first portion and the second portion. An annular gap including a linear portion and a curved portion is provided between the first portion and the second portion of the cathode. The magnet is configured to form, between the first portion and the second portion of the curved portion, a magnetic field line having a bottom inside a cross-sectional centerline of the gap.