Radiotherapy apparatus

A radiotherapy apparatus comprising:

• an irradiation unit which irradiates an irradiation object with radiation;
• and
• an imaging apparatus which takes an image of the irradiation object,
• the imaging apparatus comprising:
• a light source unit that irradiates the irradiation object with light for imaging;
• and a detection unit that detects the light through the irradiation object,
• wherein the light source unit and the detection unit are disposed at positions offset from a line connecting an isocenter of the radiation and the irradiation unit, and
• the detection unit is disposed on a reference plane which is a plane passing through the isocenter and orthogonal to the line connecting the isocenter and the irradiation unit or on a side closer to the irradiation unit than the reference plane.