NOVELTY - The fuel cell assembly (10) has reaction gas passages (52,56,46,58) for allowing at least one of reaction gases of a fuel gas and an oxidizing gas to flow in unit cells (14,16). The arrangement of gas passages is such that at least a portion of the reaction gas passages are communicated in series to each other over the unit cells.    USE - Fuel cell stack (claimed) for on-vehicle power source.    ADVANTAGE - The power generation performance of each unit cell in solid polymer electrolyte fuel cell assembly is improved efficiently. The size of cell assembly is reduced. In the cell assembly, the reaction gas is supplied to the unit cell on the upstream side so that the flow rate of reaction gas supplied is increased with a result that the humidities in the unit cells are equalized. The current density distribution in the unit cells are equalized so concentration over-potential is reduced. Water produced in the unit cells are effectively discharged by increasing the flow velocity of reaction gas supplied to the cell assembly. The drainage characteristics of whole cell assembly is improved. Since the reaction gas passages extend longer to connect several unit cells to each other, pressure drop is increased and the distribution characteristics of reaction gas between respective unit cells is improved efficiently. The workability of assembly of fuel cell stack is simplified by stacking unit cells to each other. Even if the amount of reaction gas is reduced due to electrochemical reaction, the reactions on the reaction planes in the unit cell is equalized. The contact resistance between the unit cells is reduced.    DETAILED DESCRIPTION - The solid polymer electrolyte fuel cell assembly comprises unit cells stacked to each other. Each unit cell has a unified structure (18,20) including a solid polymer electrolyte membrane disposed between an anode (26b) and cathode (24a,24b). Reaction gas passages (52,56,46,58) for allowing at least one of reaction gases of a fuel gas and an oxidizing gas to flow in the unit cells are provided in the cell assembly. The arrangement of gas passages is such that at least a portion of the reaction gas passages are communicated in series to each other over the unit cells.    INDEPENDENT CLAIMS are included for the following:    (1) fuel cell stack; and    (2) method for supplying reaction gas to solid polymer electrolyte fuel cell assembly.    DESCRIPTION OF DRAWING(S) - The figure shows flow of reaction gases and coolant in cell assembly.    Fuel cell assembly (10)    Unit cells (14,16)    Unified structures (18,20)    Cathodes (24a,24b)    Anode (26b)    Reaction gas passages (46,52,56,58)