To provide an inspection window for seeing a liquid level and the like of a pressure fluid vessel using a transparent protection plate which prevents a damage of a glass body, has resistance against liquid, vapor, or the like especially at high temperature and high pressure, has erosion resistance and corrosion resistance compared to a conventional mica plate, is hardly separated, and is hardly contaminated because silica is hardly adhered.

The inspection window of the pressure fluid vessel has a transparent protection plate made of alumina single crystal between a port portion and the body inner direction of the inspection window of the pressure fluid vessel for seeing the liquid level of the pressure fluid vessel through the glass body.