A fuel cell system has a fuel cell stack, a controller, and a cooling water channel. The controller executes the control for the temperature of cooling water flowing through the cooling water channel in the fuel cell stack. When judging the necessity to increase an oxygen concentration of air at an area near an air outlet of the air channel, the controller instructs to change a temperature of the cooling water to a condense available temperature at which water vapor involved in the air at the area near the air outlet can be condensed in order to increase the oxygen concentration in the air at the area near the air outlet. As a result, the magnitude of the electric power generated by the fuel cell stack is increased efficiently.