PURPOSE: To simplify manufacture of a gas separation board and a cooling board by fitting the portions projected from each cooling board air-tightly into the window of a reaction air manifold while fixing a cooling air manifold to cover the cooling air path exposed to the window.

CONSTITUTION: Cooling air is led to a cooling air lead-in manifold 8 then passes through the air path 4 in each cooling board 5 exposed to each window 7' in a reaction air manifold 7 to cool a stack 1 and discharged to the outside. The discharge air is cooled by a heatexchanger and recirculated to maintain the temperature of the stack 1 to the cell operating temperature through adjusting of the supply air flow. On the other hand, the reaction air is led to an auxiliary manifold 10 to enter through each conduction port 9 into a reaction air lead-in manifold 7 and passes through the reaction air path of a gas separation board 2 constituting an unit stack 1' to be discharged through conduction port 9 and auxiliary manifold 10. Reaction hydrogen to be fed from a fuel reformer will pass through a hydrogen lead-in manifold 11 and the hydrogen path of the gas separator 2 to perform cell reaction with the reaction air.