A liquid fuel mixing apparatus includes a water chamber, a fuel chamber, and a partition arranged substantially vertically between the water chamber and the fuel chamber. The partition includes a passage to permit communication between the water chamber and the fuel chamber. The water chamber includes a first port for the inflow of water and air recovered from a cathode of the fuel cell. The fuel chamber includes a third port for the inflow of unreacted fuel and CO.sub.2 from an anode of the fuel cell, a fourth port for the inflow of a liquid fuel from a liquid fuel tank, and a fifth port for the outflow of a mixed fuel to the fuel cell.