Abstract: An charged particle beam apparatus includes: a gas introduction chamber to which raw gas is introduced; a plasma generation chamber connected to the gas introduction chamber; a coil wound around an outer circumference of the plasma generation chamber and receiving a high-frequency power; an extraction electrode applying an extraction voltage to plasma discharged from a plasma aperture at an outlet of the plasma generation chamber; an ampere meter measuring a magnitude of a plasma current caused by the plasma moved out of the plasma aperture; an extraction voltage calculator calculating, based on variation in the magnitude of the plasma current measured by the ampere meter with respect to variation in the extraction voltage, an extraction voltage set value; and a controller controlling the extraction voltage based on the extraction voltage set value calculated by the extraction voltage calculator.