A charged particle beam device capable of observing a sample in an air atmosphere or gas atmosphere has a thin film for separating the atmospheric pressure space from the decompressed space. A vacuum evacuation pump evacuates a first housing; and a detector detects a charged particle beam (obtained by irradiation of the sample) in the first housing. A thin film is provided to separate the inside of the first housing and the inside of a second housing at least along part of the interface between the first and second housings. An opening part is formed in the thin film so that its opening area on a charged particle irradiation unit's side is larger than its opening area on the sample side; and the thin film which covers the sample side of the opening part transmits or allows through the primary charged particle beam and the charged particle beam.