In order to increase the number of detectable gravitational-wave sources, future gravitational-wave detectors will operate with cryogenically cooled mirrors. However, recent studies showed that cryogenic mirrors can suffer from the molecular layer formation, which introduces an additional optical loss, and the detector's performance degrades. In order to evaluate the impact of the molecular layer on future cryogenic gravitational-wave detectors, we built a cryogenic folded-cavity setup and developed an ellipsometric measurement method. The optical loss induced by the cryogenic molecular layer shows a large value even at a few nanometer thickness and can deteriorate the performance of the future cryogenic gravitational-wave detectors.