The invention provides a plasma generator which improves the service life and the plasma generation rate. A plasma generator includes: an arc chamber (12) having a plasma generation region in which plasma is generated in the inside thereof; a magnetic field generator (16) configured to apply a magnetic field (B) to the plasma generation region; and a cathode (30) configured to extend in an axial direction along an applying direction of the magnetic field (B) 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 (12) 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 (12).