PURPOSE: To stabilize the output energy of a linear accelerator by automatically controlling the level or phase of a high frequency wave according to detection of the position and the size of a beam through a beam profile monitor.

CONSTITUTION: Beam energy is measured by detecting the deflecting position of a particle beam through a beam profile monitor 38. As the distribution of energy expands when the phases of the particle beam and high frequency wave do not coincide, the low energy portion of the particle beam having passed through a deflecting electromagnet 16 for measurement of energy is greatly deflected, and the high energy portion is slightly deflected, so that the size of the beam expands horizontally. Then, phase deflection between the particle beam and the high frequency wave is measured by detecting the beam size through a beam profile monitor 38; and a controller 56 automatically controls one or both of the level and phase of high frequency wave according to the above detection. It is thereby possible to correct the phase deflection between the particle beam and the high frequency wave and a change in beam energy so that the output energy of a linear accelerator 12 can be stabilized.