The present invention provides an energy degrader (10) that can mitigate a reduction in the transmittance of a low-energy charged particle beam, and a charged particle beam irradiation system equipped therewith. The energy degrader includes a plurality of attenuating members (11A to 11G) with different amounts of energy attenuation, and the low-energy-side attenuating member (11G) with a larger amount of energy attenuation is made of a material having a higher transmittance than that of the high-energy-side attenuating member (11A) with a smaller amount of energy attenuation.
