NOVELTY - A fuel cell stack comprises stacked unit fuel cells. Each unit fuel cell has a membrane electrode assembly (20), and pair of separators (30A, 30B). An anode and a cathode interpose an electrolyte membrane in the membrane electrode assembly. The pair of separators is made of a metal plate respectively closely contacting the anode and the cathode. Each separator has protruding lines (31A) arranged vertically in a zigzag manner so as to draw a waveform, and protrude in a direction going away from the membrane electrode assembly.    USE - The fuel cell stack is used for electric power generation.    ADVANTAGE - The fuel cell has uniform relative humidity in the membrane electrode assembly; allows uniform electric power generation over the whole plane of the membrane electrode assembly, thus performing stable power generation; prevents membrane electrode assembly from drying; and prevents the reaction gas passages from being closed or blocked due to the generated water.    DETAILED DESCRIPTION - A space between the anode and the protruding lines of the separator closely contacting the anode is used as a fuel gas passage through which a fuel gas flows. A space between the cathode and the protruding lines of the separator closely contacts the cathode is used as an oxidant gas passage through which an oxidant gas flows. A part(s) of separators are each arranged in close contact with another separator so that a coolant passage through which a coolant flows horizontally is formed between both separators. A lower area of the coolant passage has a larger flow resistance than that of an upper area of the coolant passage. The lower area has a smaller pitch between waves of the waveform of the protruding lines than that of the upper area. The upper and lower areas have an individual hole for supplying the coolant into the coolant passage and an individual hole for draining the coolant drawn from the coolant passage.    DESCRIPTION OF DRAWING(S) - The figure shows front view of an anode separator as a constituent of a unit fuel cell.    Membrane electrode assembly (20)    Pair of separators (30A, 30B)    Protruding lines (31A)    Sealing parts (43, 44)