A milling apparatus is provided in which temperature rise of a treatment-object in milling treatment, especially of the substrate thereof, is prevented. In the apparatus, ionization mechanism 2 comprises casing 20d having an opening at the center portion of the face thereof opposing to substrate 5 held by substrate holder 6; a filament is placed at the position where the straight line drawn from the filament to substrate 5 is intercepted by casing 20d; and electromagnets 31, 32 are provided around ionization mechanism 2 for generating a magnetic field to produce magnetic lines extending through opening 20j to substrate 5.