PURPOSE: To enable the sure operation without influence of noise, etc., by performing the insulation between a beam duct, a vacuum container and the material directly connected to them and an electromagnetic iron core or a frame or the like as earth.

CONSTITUTION: The insulation 3 is performed between an electromagnetic iron core 1 and a beam duct 2, and a high frequency acceleration cavity 5 and the beam duct 2 connected to it is insulated from a frame 7. Furthermore, since the high frequency acceleration cavity 5 is directly connected to a high frequency power source through a coaxial pipe 6, DC breaks 8 are interposed to perform the insulation. About a signal circuit system of a beam position monitor 8 fitted to the beam duct 2, the signal is insulated from a cage material, and the direct connection to the signal system thereafter is eliminated by using the DC breaker 8 at the time of output. The influence of noise is thereby reduced to stabilize the operation.