In a charged particle beam apparatus in which observation is performed under an atmospheric pressure or a gas atmosphere substantially equivalent to atmospheric pressure, the diaphragm for isolating the atmospheric pressure space in which the sample is placed and the vacuum space inside the electron optical column is very thin for the transmission of electron beams, high. When the diaphragm is exchanged, it is necessary to adjust the diaphragm position. However, according to the prior art method, it is not possible to easily adjust the diaphragm position. In a charged particle beam apparatus having a structure in which a thin film for partitioning a vacuum atmosphere and an atmosphere or a gas atmosphere is used, a space in which the sample is placed is isolated so that the pressure of the space in which the sample is placed is kept larger than the pressure inside the casing, A detachable diaphragm for transmitting or passing the charged particle beam and a movable member for moving the diaphragm while maintaining the pressure of the space in which the sample is placed and the pressure inside the casing.
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