PROBLEM TO BE SOLVED: To solve the problem that, in a charged particle beam apparatus for observing under the atmospheric environment or a gas atmosphere of a pressure substantially equal to the atmospheric pressure, since a partitioning film for isolating an atmospheric pressure space in which a specimen is placed from a vacuum space within an electron-optical lens barrel is extremely thin, the partitioning film is highly possible to be damaged and when exchanging the partitioning film, it becomes necessary to adjust a partitioning film position, however, according to methods of the prior arts, the partitioning film position cannot be easily adjusted.SOLUTION: A charged particles beam device configured to employ a thin film for partitioning between a vacuum atmosphere, and the atmospheric environment or a gas atmosphere, comprises: a removable partitioning film through which primary charged-particle beam is transmitted or passed, isolating a space in which a specimen is placed so as to keep a pressure in the space where the specimen is placed higher than a pressure within a housing; and a movable member capable of moving the partitioning film while keeping the pressure in the space where the specimen is placed and the pressure within the housing.
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