A cathode gas cooling system provided with a heat exchanger having first internal channels into which cathode gas flows and second internal channels to which water discharged from a fuel cell is supplied and cooling cathode gas flowing through the first internal channels by latent heat of vaporization of water flowing through the second internal channels. The first internal channels and second internal channel are are respectively made independent channels inside the heat exchanger so that steam produced inside the second internal channels by heat exchange with cathode gas flowing through the first internal channels does not flow into the first internal channels.