Provided is a charged particle beam device or charged particle microscope permitting observation of even a large-sized specimen in the air atmosphere or a gaseous atmosphere.A charged particle beam device that adopts a thin film which partitions a vacuum atmosphere and the air atmosphere (or gaseous atmosphere) includes a charged particle optical lens barrel in which a charged particle optical system is stored, a housing in which a route along which a primary charged particle beam emitted from the charged particle optical lens barrel reaches the thin film is sustained in the vacuum atmosphere, and a mechanism that bears the charged particle optical lens barrel and first housing against a device installation surface. As the bearing mechanism, a housing having an opening through which a large-sized specimen is carried in or a mechanism having a shape other than the shape of the housing, such as, a post is adopted.