PURPOSE: To simplify structure, reduce the number of assembling steps for improving productivity and improve assembling accuracy and reassembling performance for excellent reliability.

CONSTITUTION: In a cylindrical vacuum vessel 1 four electrodes 2-5 are arranged around the passing route of ions while supported by posts 6, 7. This vessel 1 is composed of first and second divided walls 1a, 1b by dividing lines 15, 15 connecting an ion introducing side end to an ion leading out end, and it has a vessel form by connecting them. The post 6 is formed integrally with the first divided wall 1a while the post 7 with the second divided wall 1b.