In a charged particle beam device that performs observation of a sample under a gas environment in atmospheric pressure or pressure substantially equal to the atmospheric pressure, a diaphragm that separates an atmospheric pressure space, in which the sample is placed, and a vacuum space in an interior of an electron optical lens barrel is made very thin in order to allow an electron beam to transmit therethrough and damaged with a high possibility. Although at the time of replacing the diaphragm, it is necessary to adjust a position of a diaphragm, it is impossible to easily perform the adjustment of the position of the diaphragm by a conventional method. In a charged particle beam device with a configuration in which a thin film that separates a vacuum environment and an atmospheric environment or a gas environment is employed, a detachable diaphragm that partitions a space, in which a sample is placed, in such a manner that pressure in the space in which the sample is placed is maintained at a level larger than pressure in an interior of a housing, and that allows transmission or passage of a primary charged particle beam therethrough, and a movable member that can move the diaphragm in a state where the pressure in the space, in which the sample is placed, and the pressure in the interior of the housing are maintained as they are, are provided.