PURPOSE: To improve incidence efficiency to an after-accelerator by providing two or more position monitors in a front part of the after-accelerator, and detecting a position and proceeding angle of a beam to change an excitation quantity of an electromagnet in a beam carrier system.

CONSTITUTION: Two or more position monitors 5, 6 are installed in a front part of an after-accelerator 2 to detect a position and proceeding angle of a beam relative to a beam central path. The position and angle are regulated by changing an excitation quantity of each electromagnet 10 in a beam carrier system 3 so that the beam is injected at a desired position and angle in a incidental part of the after-accelerator 2. Incidence efficiency is thereby improved.