A focused ion beam apparatus includes a gas field ion gun unit having an emitter, an ion source gas supply unit for supplying different ion source gases to the emitter, a heater for heating the emitter, and an extraction electrode. A storage section stores, for each gas of a plurality of different types, set values of emitter temperature, gas pressure, extraction voltage to be applied to an extraction electrode, image contrast and image brightness. An input section selects and inputs one of the gas types. A control section reads, from the storage section, the set values of emitter temperature, gas pressure, extraction voltage, image contrast and image brightness, which correspond to the input gas type, and sets a heater, a gas control section, a voltage control section, and an adjustment section for the contrast and brightness of the image.