PURPOSE: To prevent the deterioration of a detector and to considerably lengthen the service life, by providing a board for shielding the secondary electron openably/ closably at the front face of a secondary electron detector.

CONSTITUTION: A secondary electron detector 10 having a scintillator 11 is fixed to the body, while a shielding board 12 is supported perpendicularly at the inner end of a shaft rotatably supported by the body, and the front face of the secondary electron detector 12 is covered by the shielding board 12. A handle 13 is fixed at the outer end of the shaft and sealed through an O ring 14 air-tightly between the body. Consequently when not using the secondary electron detector, it can be protected from deterioration by shielding resulting in the considerably lengthening of the service life.