The effect of the temperature drop by phase transition on mass transfer in prevaporation module and the phenomenon of the apparent concentration polarization by the change of flow rate were discussed in term of the heat supply ratio defined in this study. The heat supply ratio indicates the relative amount of heat supplied from retentate form phase transition of permeate. With experiments, the mass transfer was decreased with increasing of the feed temperature for phase transition of permeate. With experiments, the mass transfer was decreased with increasing of the feed temperature and the heat supply ratio because of higher temperature drop. As the feed flow rate was decreased, the temperature drop was increased and the mass transfer was decreased. This phenomenon has a similarity of the concentration polarization in the case of other membrane processes without phase transition. It was shown that the effects of the temperature drop on mass transfer were not negligible from the comparison of the proposed formular for the concentration polarization.