Provided is a charged particle beam apparatus or charged particle microscope capable of observing an observation target sample in an air atmosphere or a gas environment without making significant changes to the configuration of a conventional high vacuum charged particle microscope. In a charged particle beam apparatus configured such that a thin film (10) is used to separate a vacuum environment and an air atmosphere (or a gas environment), an attachment (121) capable of holding the thin film (10) and whose interior can be maintained at an air atmosphere or a gas environment is inserted into a vacuum chamber (7) of a high vacuum charged particle microscope. The attachment (121) is vacuum-sealed and fixed to a vacuum partition of the vacuum sample chamber. Image quality is further improved by replacing the atmosphere in the attachment with helium or a light-elemental gas that has a lower mass than atmospheric gases such as nitrogen or water vapor.