To improve accuracy in irradiating position to a focus, of a radiation beam in an initial stage, and to reduce the exposure to the normal tissue around the focus, by removing a beam from a path to make it collide against a prescribed stopper during a period when an irradiation inhibiting signal is output, and returning the same to the normal path during a period when an irradiation enabling signal is output.
The energy of an electron beam is consumed and absorbed when the electron beam collides with a beam stopper 12. When a signal of an irradiation enabling/inhibiting signal output circuit 16 is changed from the irradiation inhibiting signal 21a to the irradiation enabling signal 12b, a coil energization current control circuit 11a returns the electric current of a circular trim coil 7 to the original low value. Thereby the electron beam 9 is returned to the normal orbit for the radiotheraphy, to be output to an external, to apply the beam to a focus. When the signal from the irradiation enabling/inhibiting signal output circuit 16 is changed to the irradiation inhibiting signal 21a, the coil energization current control circuit 11a increases the current of the circular trim coil 7 again, thereby the electron beam 9 is collided with the beam stopper 12, and the output to the external is stopped.
NAGAOKA TAKAYUKI
SHINOHARA MASARU
