In the case of a conventional gas field ionization ion source, it was not possible to carry out an analysis with a high S/N ratio and a high-speed machining process because the current amount of an ion beam is small. In view of these problems, the present invention has been devised, and its object is to obtain a large ion beam current, while suppressing a probability of damaging an emitter electrode. The present invention is characterized by a process in which an ion beam is emitted at least in two operation states including a first operation state in which, when a first extraction voltage is applied, with the gas pressure being set to a first gas pressure, ions are emitted from a first ion emission region at the apex of the emitter electrode, and a second operation state in which, when a second extraction voltage that is higher than the first extraction voltage is applied, with the gas pressure being set to a second gas pressure that is higher than the first gas pressure, ions are emitted from a second ion emission region that is larger than the first ion emission region.