An ion source includes a plasma generation chamber, at least one filament disposed inside the plasma generation chamber, at least one electrode disposed so as to be opposed to the plasma generation chamber, and configured to extract out an ion beam from the plasma generation chamber, and a plurality of permanent magnets disposed outside the plasma generation chamber, and configured to form cusped magnetic fields inside the plasma generation chamber, and a deposition preventive plate disposed parallel with an inner surface of a wall of the plasma generation chamber. The deposition preventive plate has recesses which are formed at such positions as to be opposed to the respective permanent magnets with the wall of the plasma generation chamber interposed in between.