To prevent damages to a lid opening and closing mechanism.

A limiter switch 31 is attached to a side face of a sample room 2 except for the lid 14. The limiter switch 31 comprises a switch body 33 having a switch 32 mounted on the surface of the switch body 33, and a spring plate 36 having a contact ball 35 fixed at its tip installed on a shaft 34 fixed to the switch body 33. An L shaped section of a metal body 37 is mounted on a side of the lid 14 directly above the limiter switch 31. The limiter switch 31 outputs a switch-on signal when the contact ball 35 fixed at the tip of the spring plate 36 is separated from the metal body 37 and while the contact ball 35 fixed at the tip of the spring plate 36 is touching the metal body 37 the limiter switch 31 does not output any signal. On the inner face of the lid 14 an O ring 38 groove having a depth smaller than the diameter of the cross- section of the O ring 38 is provided and the O ring 38 installed into the groove for vacuum sealing.