An energy degrader includes: an attenuation member that attenuates energy of entering charged particles to a different amount of energy according to an incident position of the charged particles an energy adjustment drive unit that drives the attenuation member in a first axial direction in order to change the incident position of the charged particles and a beam diameter adjustment drive unit that drives the attenuation member in a second axial direction different from the first axial direction in order to adjust the beam diameter of the charged particles.
