1. A charged particle beam extraction system for extracting a charged particle beam through an irradiation device, said charged particle beam extraction system comprising: an accelerator for cyclically performing patterned operation including steps of introducing, accelerating and extracting the charged particle beam; an amount-of-extracted beam adjusting unit for changing an amount of the charged particle beam extracted from said accelerator; and a control unit for controlling said amount-of-extracted beam adjusting unit such that when extraction of the charged particle beam is stopped at least once during the extraction step of said accelerator, the amount of the charged particle beam extracted from said accelerator in one cycle is held substantially at a setting value of the amount of extracted beam.