Radiotherapy is known which cures a patient by irradiating therapeutic radiation to an affected part of tumor.  The radiotherapy system which carries out radiotherapy is provided with a therapeutic radiation irradiating unit which irradiates the therapeutic radiation, a sensor which measures a position of the affected part of the patient, and a drive unit which moves the therapeutic radiation irradiating unit such that the therapeutic radiation is irradiated to the measured position.  According to such a radiotherapy system, even when the affected part moves with a breathing operation of the patient, the therapeutic radiation can be irradiated surely to the affected part.  In such radiotherapy, it is demanded that the therapy effect is high.  Also, it is demanded that a quantity of the therapeutic radiation irradiated to normal cells is smaller than the radiation quantity is irradiated to cells in the affected part.  For this reason, it is demanded that the radiotherapy system irradiates the therapeutic radiation to the affected part in a high accuracy and moves the therapeutic radiation irradiating unit in a high accuracy.  Moreover, in the radiotherapy system, it is demanded that the responsibility of the drive unit is high and moreover that the operation of the drive unit is stable. 
In JP 2004-65808A, the radiotherapy system is disclosed which forms a radiation field from a wide radiation field to a minute unshaped radiation field, which is possible to reduce an irradiation time and appropriately irradiate according to movement of the body of the patient, and which is possible to have a small size.  The radiotherapy system is provided with an electron beam generating source, a deflection electromagnet which changes a direction of the electron beam, a vacuum window configured to pass the electron beam while holding the vacuum state, a scattering foil configured to scatter the electron beam, a target which converts the electron beam into an X-ray, a flattening filter circuit configured to make dose distributions of the electron beam and the X-ray uniform in the irradiation plane, collimators which collimate the electron beam and an X-ray, an irradiation head having dosimeters configured to measure the doses of the X-ray and the electron beam, and a gantry arm which supports the irradiation head.  The radiotherapy system further contains a rotating unit couples the electron beam source and the deflection electromagnet by a vacuum rotary joint, and swings the irradiation head with respect to an axis parallel to a gantry arm rotation axis and passing a virtual source position. 
In JP 2006-21046A, a radiotherapy system is disclosed which the condition of a therapeutic field can be monitored in real time even during radiotherapy.  The radiotherapy system is provided with an O-type gantry, a radiation irradiating head which is provided rotatably for the O-type gantry, to irradiate a therapeutic radiation to the therapeutic field, an X-ray source which is provided movably for the O-type gantry, to irradiate a diagnostic X-ray to the therapeutic field, and sensor arrays which are provided movably for the O-type gantry, to generate diagnostic images by detecting the diagnostic X-rays passing through the sample.  The sensor arrays are provided on symmetrical positions with respect to the radiation irradiating head, to move on the O-type gantry in conjunction with the movement of the radiation irradiating head, and the X-ray source moves in response to the movement of the sensor arrays.