PURPOSE: To introduce a sufficient amount of gas into a sample chamber without any degree of vacuum lowered inside the sample chamber and improve the durability of an electron beam source by disposing a thin film allowing penetration of a charging beam there through and intercepting a gas between the charging beam source and the sample chamber.

CONSTITUTION: An electron beam emitted from an electron beam source 20 penetrates a thin film 40 and is introduced into a chamber 10, but a gas introduced into the chamber 10 is intercepted by the thin film 40, so that the gas can not flow into the electron beam source 20. Accordingly, a sufficient amount of gas may be introduced into the sample chamber 10 without any degree of vacuum lowered inside the electron beam source. In addition, it is made possible to increase an amount of current of the electron beam reaching a sample 11. Furthermore, since no gas flows into the electron beam source 20, the corrosion and the like of the electron beam source 20 may be prevented regardless of the kind of gas in use, so that the durability of the electron beam source 20 may be improved.