PROBLEM TO BE SOLVED: To perform accurate radiation irradiation while performing moving body tracking, with a small-sized and light-weight device configuration.SOLUTION: A radiotherapy system including an electron gun for generating an electron beam and a target for converting the generated electron beam to a radiation comprises: a first collimator that arranges the target 4 inside the first collimator to prevent the radiation from leaking outside and a second collimator that is arranged inside the first collimator so as to be in a state with a gap and allows the radiation to pass in the direction of the second collimators axis, the gap being used for oscillating the second collimator.SELECTED DRAWING: Figure 2