NOVELTY - The cell (10) has a membrane electrode assembly (16) including a solid polymer electrolyte membrane (50) provided between electrodes (52) along stacking direction (A). A separator (18) includes a water accumulation portion comprising a buffer portion connected to a reactant/fuel gas channel (42) through which reactant gas is to flow along a power generation surface of the assembly. A bypass channel bypasses the gas channel. The assembly includes a water impermeable layer (56), which is arranged outside of a power generation region of the assembly and faces the accumulation portion.    USE - Solid polymer electrolyte fuel cell i.e. power generation cell, for a vehicle.    ADVANTAGE - The assembly includes the water impermeable layer arranged on an oxidant gas channel side of the separator in a region facing the accumulation portion and in a region facing embossed portions of an inlet buffer portion and the outlet buffer portion, so that the solid polymer electrolyte membrane is isolated from the accumulation portion, thus reducing degradation of the membrane in a reliable manner due to metal ions dissolved in water when the metal ions are leached into the water in the inlet buffer portion, where the water tends to accumulate the metal ions to be blocked by the water impermeable layer and does not contact the membrane.    DETAILED DESCRIPTION - The electrodes include cathode electrodes. The buffer portion includes inlet and outlet buffer portions. The power generation region is an electrode catalyst layer. The solid polymer electrolyte membrane is a polymer ion-exchange membrane. The cell utilizes fuel gas that is hydrogen-containing gas. The cell is a power generation cell (12). The water accumulation portion is a lowermost channel groove.    DESCRIPTION OF DRAWING(S) - The drawing shows an exploded perspective view of a power generation cell of a fuel cell.    Stacking direction (A)    Solid polymer electrolyte fuel cell (10)    Power generation cell (12)    Membrane electrode assembly (16)    Separators (18, 20)    Reactant gas channel (42)    Solid polymer electrolyte membrane (50)    Cathode electrodes (52)    Anode electrodes (54)    Water impermeable layer (56)