Fuel cell system

A fuel cell system which includes: a fuel cell having an electrolyte membrane; a tank for storing water for humidifying the electrolyte membrane; a pump for circulating the water through the tank and the fuel cell; a sensor for sensing amount of water in the tank; and a controller for controlling operation of the fuel cell system. The controller changes I-V characteristic of the fuel cell to shift operation point thereon depending on the amount of water sensed by the sensor.