In an accelerating tube which uses a conductive insulator, there is a possibility that the dopant concentration on a surface of the conductive insulator becomes non-uniform so that the surface resistance of the conductive insulator becomes non-uniform. Accordingly, a circumferential groove is formed on the inner surface of the conductive insulator accelerating tube in plural stages, and metal is metalized along inner portions of the grooves. When the resistance of a specific portion on the surface of the accelerating tube differs from the resistance of an area around the specific portion, the potential of the metalized region on the inner surface of the accelerating tube becomes a fixed value and hence, the potential distribution on the inner surface of the accelerating tube in the vertical direction can be maintained substantially equal without regard to the circumferential direction.