A focused ion beam apparatus has an ion source chamber in which is disposed an emitter for emitting ions. The surface of the emitter is formed of a precious metal, such as platinum, palladium, iridium, rhodium or gold. A gas supply unit supplies nitrogen gas to the ion source chamber so that the nitrogen gas adsorbs on the surface of the emitter. An extracting electrode is spaced from the emitter, and a voltage is applied to the extracting electrode to ionize the adsorbed nitrogen gas and extract nitrogen ions in the form of an ion beam. A temperature control unit controls the temperature of the emitter.