A focused ion beam apparatus includes an ion gun unit having an emitter tip, a gas supply unit that supplies gas to the tip, and an ion source gas supply source. An extracting electrode ionizes the gas adsorbed onto the surface of the tip and extracts ions by applying a voltage between the extracting electrode and the tip. A cathode electrode accelerates the ions toward a sample. An aperture member has an opening that passes therethrough a part of the ion beam ejected from the ion gun unit, and a lens system focuses the ion beam onto the sample.