We have developed an experimental apparatus for in situ observation of a physisorption system on a metal surface by an ellipsometry and an electron diffraction. In order to observe a physisorption system and a thick insulator film, we have developed an extremely low-current low energy electron diffraction (XLEED) apparatus equipped with a microchannel plate and a position-sensitive detector in place of a phosphor screen in an ordinary LEED optics. The ellipsometric adsorption isotherms of Xe/Ag (111) at substrate temperatures between 50 and 80 K were obtained in the wide pressure range between 10(-7) and 1 Pa. We observed the surface structure by XLEED while monitoring the layer growth by the ellipsometry from a submonolayer to a thick film where the equilibrium pressure was nearly equal to the bulk saturation vapor pressure. The Xe overlayer of a sufficiently thick film has clear (111) structure which keeps relative orientation to the Ag (111) substrate.