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.本發明提供一種可調節帶電粒子的能量的衰減量,並且可調節帶電粒子束的射束束徑擴大之降能器及具備該降能器之帶電粒子照射系統。在降能器(10)中,設為具備衰減材料(11)之結構,使入射至衰減材料之帶電粒子減速來衰減能量。設置向第1軸線方向X驅動衰減材料(11)之能量調整用驅動手段,並改變帶電粒子相對於第1衰減材料之入射位置,藉此可調節帶電粒子的衰減量。另外,設置向第2軸線方向Y驅動衰減材料(11)之射束束徑調整用驅動手段,並改變衰減材料(11)在帶電粒子束的前進方向上之位置,藉此調節射束束徑擴大。10‧‧‧降能器11‧‧‧衰減材料11a‧‧‧入射面11b‧‧‧傾斜面12‧‧‧衰減材料(第2衰減材料)12a‧‧‧射出面12b‧‧‧射出面13‧‧‧降能器的出口
