Patent Number: 
Section: claims

1. A particle-beam energy changing apparatus comprising:a first energy changing unit and a second energy changing unit each having a particle-beam energy attenuator and arranged so that a particle beam passes through the first energy changing unit and the second energy changing unit, to change energy of the passing particle beam by varying thicknesses of the attenuators; andan energy-change substitution control unit which controls the first energy changing unit and the second energy changing unit,wherein a maximum attenuation amount by the first energy changing unit is set smaller than a maximum attenuation amount by the second energy changing unit, andthe energy-change substitution control unit is configured to perform a control of changing energy of the particle beam at least by changing an attenuation amount by the first energy changing unit when energy of the particle beam is controlled to change, anda substitution control that substitutes an attenuation amount by the first energy changing unit and an attenuation amount by the second energy changing unit for each other by changing the attenuation amount by the first energy changing unit and the attenuation amount by the second energy changing unit so that a total attenuation amount of the attenuation amount by the first energy changing unit and the attenuation amount by the second energy changing unit remains unchanged when energy of the particle beam after passing through the particle-beam energy changing apparatus is controlled to be constant. 2. The particle-beam energy changing apparatus according to claim 1, wherein a maximum speed of changing an attenuation amount by the first energy changing unit is faster than a maximum speed of changing an attenuation amount by the second energy changing unit. 3. The particle-beam energy changing apparatus according to claim 1, wherein values of target attenuation amounts to be imparted to the particle beam are given as time series data of the target attenuation-amount values, and instructions to the first energy changing unit and the second energy changing unit are generated on the basis of two data sets separated in frequency from the time series data of the target attenuation-amount values. 4. The particle-beam energy changing apparatus according to claim 1, wherein correction of energy of the particle beam is performed by varying an attenuation amount by the first energy changing unit. 5. A particle beam therapy system comprising a particle-beam energy changing apparatus according to claim 1. 6. A method of changing energy of a particle beam, in which using a first energy changing unit and a second energy changing unit each having a particle-beam energy attenuator, energy of a particle beam passing through both units is changed by varying thicknesses of the particle-beam energy attenuators, wherein a maximum attenuation amount by the second energy changing unit is set larger than a maximum attenuation amount by the first energy changing unit, the method of changing energy of a particle beam comprising:changing the particle beam energy at least by changing an attenuation amount by the first energy changing unit when the particle beam energy is controlled to change; andsubstituting an attenuation amount by the first energy changing unit and an attenuation amount by the second energy changing unit for each other, when energy of the particle beam after passing through the first energy changing unit and the second energy changing unit is controlled to be constant, by changing the attenuation amount by the first energy changing unit and the attenuation amount by the second energy changing unit so that a total attenuation amount of the attenuation amount by the first energy changing unit and the attenuation amount by the second energy changing unit remains unchanged.