An ion source includes a vacuum chamber having a cooling mechanism, an ion generation container for reacting an ionized gas with an ion material so as to generate ions, an extraction electrode for extracting ions generated in the ion generation container and generating an ion beam, and a shielding member provided inside and in the vicinity of an inner wall of the vacuum chamber, and having a main body made of a conductive metal for blocking deposition of an insulating material on the inner wall (10d) of the vacuum chamber. The main body of the shielding member has a plurality of protruding support portions that is in contact with the inner wall of the vacuum chamber for supporting the main body in a manner such that the main body is fitted at a distance from the inner wall of the vacuum chamber.