To clarify the mechanism of coupling phenomena such as heat, mass (gas, water droplet) and electric charge transfer in a single-cell polymer electrolyte fuel cell (PEFC), in-situ temperature measurement under power generation was carried out by use of thermography, since temperature distribution plays an important role in coupling phenomena. By using a single cell filled with an observation window, the in-plane temperature distribution on the opposite side to the gas channel of the separator could be measured when the positions of the gas inlet and outlet of the cell and the gas flow rate at the inlet were changed. Gas flow rate at the inlet and outlet of the cell, gas temperature and dew point at the inlet of the cell, current and voltage were measured at the same time, and their relationship with temperature distribution was investigated. It was found that the high temperature region changed when the position of gas inlet and outlet of cell at cathode was changed. When the gas flow rate at the inlet was increased over that of normal operation, the temperature over the whole observation area decreased with increasing gas flow rate at the inlet due to the cooling effect of heat convection. When the stoichiometric ratio of H(2) was set at 1.00, the power generation performance and the in-plane temperature distribution did not change. However, when the stoichiometric ratio of O(2) was set at 1.00, a voltage drop was recognized. In addition, the high temperature region shifted from the area around the gas outlet to the area around the gas inlet. In-plane temperature distribution in the observation area was influenced by the gas flow in the channel and the gas flow pattern of the separator at the cathode. Thus, it became clear that the supply method of oxygen dominated the in-plane temperature distribution and reaction distribution.