S.Miyoki, T.Uchiyama, T.Tomaru, D.Tatsumi, H.Ishizuka, M.Ohashi, K.Kuroda, A.Ueda, T.Suzuki, N.Sato, T.Haruyama, A.Yamamoto, T.Shintomi.

LCGT (Large Scale Cryogenic Laser Interferometer), which is proposed as one of a Japanese future project, is characteristic to use cryogenic mirrors to reduce thermal noise. Such cryogenic mirrors, however, are worried to be contaminated with absorbed molecules. In our presentation, we will talk about the long term reflectance change of the cryogenic mirrors by monitoring a finesse of a Fabry-Perot cavity composed of them.