The in-situ observation of friction surfaces is conducted under the wet conditions in this report. Three kinds of friction materials removed from the commercial pads are used for the experiment. The friction coefficient and the friction behavior of each pad material are investigated. The difference of friction phenomenon between the cast iron and the glass used as the disc rotor is clarified, and the validity of replacing to the glass rotor for the observation is considered. Moreover, it is speculated that the coefficient of friction varies correspondingly to the mixture of debris and moisture, and its moisture ratio.