There are provided an energy degrader that can adjust the amount of energy attenuation of charged particles and can adjust the increase in beam diameter of a charged-particle beam, and a charged particle irradiation system including the energy degrader. The energy degrader (10) includes an attenuation member (11), and attenuates energy by decelerating charged particles entering the attenuation member. The energy degrader includes energy adjustment drive means for driving the attenuation member 11 in a first axial direction X, and can adjust the amount of attenuated energy of the charged particles by changing the incident position of the charged particles on the first attenuation member. In addition, the energy degrader includes a beam diameter adjustment drive means for driving the attenuation member (11) in a second axial direction Y, and can adjust the increase in beam diameter by changing the position of the attenuation member 11 in a traveling direction of a charged-particle beam.