In order to provide an ion beam apparatus excellent in safety and stability even when a sample is irradiated with hydrogen ions, the ion beam apparatus includes a vacuum chamber, a gas field ion source that is installed in the vacuum chamber and has an emitter tip, and gas supply means for supplying a gas to the emitter tip. The gas supply means includes a mixed gas chamber that is filled with a hydrogen gas and a gas for diluting the hydrogen gas below an explosive lower limit.