A plasma generator includes: an arc chamber having a plasma generation region in which plasma is generated in the inside thereof; a magnetic field generator configured to apply a magnetic field to the plasma generation region; and a cathode configured to extend in an axial direction along an applying direction of the magnetic field applied to the plasma generation region and provided with a cathode cap that emits thermal electrons at a front end thereof. The cathode cap protrudes toward the inside of the arc chamber in the axial direction and has a shape of which a width in the radial direction perpendicular to the axial direction becomes smaller toward the inside of the arc chamber.