A radiation therapy system, comprising: a radiation source ; a rotation mechanism that supports the radiation source and rotates the radiation source around an isocenter (C0); a couch that places a therapy target site of a patient (B) at the isocenter (C0); a head swing mechanism that is disposed between the radiation source and the rotation mechanism and that swings an irradiation axis (Sc) of a radiation emitted from the radiation source by swinging the radiation source ; and a control unit configured to control the radiation source , the rotation mechanism , and the head swing mechanism , such that the control unit holds the head swing mechanism in a state where the irradiation axis (Sc) of the radiation of the radiation source is shifted from the isocenter (C0) in a predetermined direction by a predetermined amount, and rotates the radiation source by the rotation mechanism while emitting the radiation from the radiation source while maintaining the state of the head swing mechanism , and a direction in which the control unit shifts the irradiation axis (Sc) from the isocenter (C0) includes a component in a rotation axis direction of the rotation mechanism , and the system being characterised in that the control unit is configured such that the control unit shifts the irradiation axis (Sc) from the isocenter (C0) in a positive direction in the rotation axis direction, rotates the radiation source by the rotation mechanism in this state, and then shifts the irradiation axis (Sc) from the isocenter (C0) in a negative direction in the rotation axis direction, and rotates the radiation source by the rotation mechanism in this state. a radiation source ; a rotation mechanism that supports the radiation source and rotates the radiation source around an isocenter (C0); a couch that places a therapy target site of a patient (B) at the isocenter (C0); a head swing mechanism that is disposed between the radiation source and the rotation mechanism and that swings an irradiation axis (Sc) of a radiation emitted from the radiation source by swinging the radiation source ; and a control unit configured to control the radiation source , the rotation mechanism , and the head swing mechanism , such that the control unit holds the head swing mechanism in a state where the irradiation axis (Sc) of the radiation of the radiation source is shifted from the isocenter (C0) in a predetermined direction by a predetermined amount, and rotates the radiation source by the rotation mechanism while emitting the radiation from the radiation source while maintaining the state of the head swing mechanism , and a direction in which the control unit shifts the irradiation axis (Sc) from the isocenter (C0) includes a component in a rotation axis direction of the rotation mechanism , and the system being characterised in that the control unit is configured such that the control unit shifts the irradiation axis (Sc) from the isocenter (C0) in a positive direction in the rotation axis direction, rotates the radiation source by the rotation mechanism in this state, and then shifts the irradiation axis (Sc) from the isocenter (C0) in a negative direction in the rotation axis direction, and rotates the radiation source by the rotation mechanism in this state. <EOS>