A cooling system includes a radiator for radiating heat of a coolant heated by a fuel cell, and a first main pipe line and a second main pipe line which allow coolant to circulate between the fuel cell and the radiator. A thermostat is located midway through the second main pipe line and adapted to switch the circulation path for the coolant, and a bypass pipe line. The radiator includes a first tank connected to the first main pipe line, a second tank connected to the second main pipe line, and a cooling portion provided between the first tank and the second tank and adapted to perform heat exchange with the outside air. The thermostat is connected to the first tank by the bypass pipe line. The resulting configuration facilitates pipe line routing and increases the freedom of the layout of components.