A focused ion beam apparatus includes an ion gun unit having an emitter tip, a gas supply unit including an ion source gas nozzle configured to supply 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, and a gun alignment electrode positioned on the side of the sample with respect to the ion gun unit and adjusts the direction of irradiation of the ion beam ejected from the ion gun unit. A lens system includes a focusing lens electrode and an objective lens electrode to focus the ion beam onto the sample.