  The detection of gravitational-waves provides us with a deep insight into the
universe. In order to increase the number of detectable gravitational-wave
sources, several future gravitational-wave detectors will operate with
cryogenic mirrors. Recent studies, however, showed residual gas molecules
inside the vacuum chamber adhere to the cryogenic mirror surface and form a
molecular layer which grows with time. The growing molecular layer introduces
an additional optical loss in the mirror which can decrease the detector's
performance. We theoretically estimate the optical loss by the molecular layer
in a cryogenically operated gravitational-wave detector. The impacts on a
cryogenic gravitational-wave detector is discussed based on the results of
optical loss estimation.
