Abstract: The efficiency of the fuel cell system is increased. This is enabled by controlling a bypass amount of cooling liquid for a conductivity reduction device such as an ion removal device in response to the operating state of the fuel cell system before the conductivity exceeds the permitted limiting value. The device is provided with a recirculation passage 10 which recirculates cooling liquid between a fuel cell 2 and a heat exchanger 5, a bypass passage 11 having an ion removal filter 4 therein, and a control device 1 which controls solenoids 6, 7 to increase a bypass ratio when the conductivity exceeds the permitted value.