PURPOSE: To realize acceleration with high efficiency and stabilize output energy by detecting an excessive high frequency level to control the phase of high frequency wave applied to an acceleration tube.

CONSTITUTION: The level detectors 38, 40 to detect the level of an excess high frequency wave outputted from accelerators 13-1, 13-2 and the phase shifters 34, 44 to control the phase of high frequency wave supplied to acceleration tubes 13-1, 13-2 according to the detected level are installed. If the phases of a particle beam and a high frequency wave do not coincide with each other, the part to be used for acceleration of the high frequency energy applied to the acceleration tubes 13-1, 13-2 decreases, so that the level of excessive high frequency level increases. Then, this excessive high frequency level is detected to control the phase of high frequency wave applied to the acceleration tubes 13-1, 13-2. It is thereby possible to make the phases of the high frequency wave and the particle beam in the acceleration tubes 13-1, 13-2 coincide, also realize acceleration with high efficiency and stabilize the output energy of a linear accelerator.