FUEL CELL SYSTEM

When a time period from a stop to a start of a fuel cell system exceeds a predetermined time period, a controller of the fuel cell system obtains a first electrical conductivity of a cooling medium that is placed from a radiator to before a connecting location of one end portion in a cooling medium circulation flow path and a second electrical conductivity of the cooling medium that is placed on a downstream side of an ion exchanger in a bypass flow path, and uses the obtained first electrical conductivity and second electrical conductivity and a predetermined target electrical conductivity of a supply cooling medium to control the operation of a flow dividing valve such that the electrical conductivity of the supply cooling medium becomes equal to or less than the target electrical conductivity and thereby regulate a flow rate ratio.