The present invention is provided to enable a detailed inspection of a specimen and preventing a distortion of an observation image even when a specimen containing an insulating material is partially charged. For a scanning ion microscope utilizing a gas field ionization ion source, a thin film is disposed between an ion optical system and a specimen, and an ion beam is applied to and transmitted through this thin film in order to focus a neutralized beam on the specimen. Furthermore, an electrode for regulating secondary electrons discharged from this thin film is provided in order to eliminate mixing of noises into an observation image.