The aim of this study is to construct a simple heat-transfer model to present the temperature of the interface between the polymer electrolyte membrane (PEM) and the catalyst layer at the cathode, i.e., the reaction surface, in a single cell of polymer electrolyte fuel cell (PEFC). The model is based on the temperature data of the separator measured by thermograph in a power-generation experiment. In addition, this study also aims to investigate the effect of the operation condition on the temperature of the reaction surface using the heat-transfer model developed. The heat-transfer model is constructed by assuming multi plate heat transfer for components of a single cell of PEFC. In this model, the temperature of the reaction surface under the rib of separator and that under the gas channel of the separator are assumed to be the same. The result shows that the temperature of the reaction surface is higher with increasing gas channel pitch. The impact of the flow rate of the supply gas on the temperature of the reaction surface is small when O-2 is used as the cathode supply gas. When air is used as the cathode supply gas, the temperature of the reaction surface is higher than that when O-2 is used. The temperature of the reaction surface at the inlet is lower than that at the middle and outlet of the cell. This study can explain these temperature characteristics under several conditions by power-generation performance and energy conversion of the fuel cell.