Charged particle beam irradiation system and method of extracting charged particle beam

A charged particle beam irradiation system includes an accelerator for accelerating a charged particle beam, a beam irradiation apparatus having a beam energy modulator and arranged for irradiating an object with the charged particle beam extracted from the accelerator and having passed the beam energy modulator, which is rotated and whose thickness in the axial direction differs in the rotational direction, and a controller for controlling the extraction intensity of the charged particle beam extracted from the accelerator, while the charged particle beam is being extracted, on the basis of the rotational angle of the beam energy modulator.