To clean a back surface of an opening and the downstream thereof, and to prevent the generation of oxide, and to easily eliminate the oxide even if the oxide is generated by providing each means for flowing the radical from a light source side of a lens barrel for housing an optical system to a sample chamber side and in the opposite direction, and flowing the radical in double ways.

A radical flow passages is formed in double way from an electron gun 1 side to a sample chamber 19 side and from the sample chamber 19 side to the electron gun 1 side by switching a gate valve 2, a vacuum valves 21, 22 and valves 10, 11, 27, 28. The oxygen radical generated in a discharge chamber 12 by the gas supplied by a gas cylinder 11 and generated in the chamber 23 by the gas supplied by a gas cylinder 24 in order are led into a lens barrel by an air release pump 20 and an air release pump 3 in order. The gas cylinder 13, 24 are replaced with hydrogen gas cylinder, and reduction is performed from the electron gun 1 side and from the sample chamber 19 side in order by the hydrogen radical generated in the discharge chambers 12 and 23 in order by operating each valve. Each optical system part coated by platinum is hard to be formed with an oxide film, and even if the oxide film is formed, it can be easily deoxidized.