PROBLEM TO BE SOLVED: To provide a device for adjusting energy of a neutron which has a simple device configuration and is easy to operate.SOLUTION: A plurality of moderator material containers 11,12,13 are disposed on a neutron generating side of a target generation device 2, and heavy water is selectively introduced in the moderator material containers 11,12,13 so that a neutron is slowed down when passing. The moderator material containers 11,12,13 are connected to a storage tank 14 through piping 17. The heavy water is introduced with the use of a pump 15. An epithermal neutron is converted into a thermal neutron when passing through the moderator material containers 11-13, and is radiated on a patient P on a bed B. As a result, an energy switching operation is performed with ease even when many patients are treated, and the device can be easily operated even by a doctor or a radiation engineer.