There is provided a particle beam therapy system that can rapidly change beam energy without increasing the size of a deflection electromagnet even in the case where the number of required beam-energy changes is large.There is provided a plurality of beam energy changing units each provided with a beam energy attenuation unit; a beam is deflected in such a way as to sequentially passes through the plurality of beam energy changing units; while a beam passes through one of the beam energy changing unit, the beam energy attenuation amount of another beam energy changing unit is changed.