A charged particle beam irradiation system and a charged particle beam extraction method which can prevent erroneous irradiation of a charged particle beam in the direction of advance of the charged particle beam. The system and method are featured in stopping supply of an ion beam to one or more of a plurality of angle zones in each of which a target dose is attained, the angle zones being formed by dividing an RMW in a rotating direction thereof, and in allowing the supply of the ion beam to one or more other angle zones in each of which a target dose is not yet attained. The invention can easily adjust beam doses at various positions in an affected part of the patient body in the direction of advance of the ion beam, and can greatly reduce the probability of erroneous irradiation that the beam dose becomes excessive or deficient at the various positions within the affected part of the patient body in the direction of advance of the ion beam.