PURPOSE: To mount/demount an alignment orifice in a narrow space by providing a recessed section which can position the alignment orifice in an electron beam passage, combining a screw cylinder with a cylinder section, and thereby holding a structure combined with insulators in a space interspaced with a base.

CONSTITUTION: One of two insulators 5 and 5 which hold an alignment orifice 1 in an insulation manner, is fitted in the recessed section 6a of a cap 6 and the other insulator 5 is pushed onto the opening end periphery of the opening 7a of a base 7. In the cap 6 the screw section 6d of its cylinder section 6c is screwed to the screw section 7c of a screw cylinder 7b on the base 7. This arrangement allows both the orifice 1 and each insulator 5 to be screwed onto the base 7 while the relation in position between the orifice 1 and each insulator 5 is being kept by the cap 6. In this case, the misaligned orifice 1 is fixed vertically and horizontally, and the center axis of the misaligned orifice 1 can concurrently be made to coincide with the center axis of electron orbit 3. By this constitution, the misaligned orifice 1 can be manually mounted onto the base 7 in a simple manner without using any tool even if a space is small.