A focused ion beam apparatus has a gas injector provided with a plurality of gas supply pipes each having a nozzle at one end for ejecting a gas and a housing in which the plurality of gas supply pipes are slidably accommodated. The housing has a tip portion with a hole provided at a terminal end thereof and the hole has a size set so that only one of the nozzles can extend therefrom. In addition, the housing has a tapered nozzle guide portion for guiding a selected nozzle from a retracted position at which the selected nozzle does not extend from the hole to an extended position at which the selected nozzle extends from the hole.
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