Fuel cell separators capable of suppressing variation in pressure loss

A fuel cell stack formed by stacking a plurality of power generation units, the power generation units each comprising at least first and second electrolyte electrode assemblies, and formed by stacking the first electrolyte electrode assembly on a first metal separator, a second metal separator on the first electrolyte electrode assembly, the second electrolyte electrode assembly on the second metal separator, and a third metal separator on the second electrolyte electrode assembly, the first and second electrolyte electrode assemblies each comprising a pair of electrodes, and an electrolyte interposed between the electrodes, first to fourth reactant gas flow fields for allowing predetermined reactant gases to flow along power generation surfaces being formed between the first metal separator and the first electrolyte electrode assembly, between the first electrolyte electrode assembly and the second metal separator, between the second metal separator and the second electrolyte electrode assembly, and between the second electrolyte electrode assembly and the third metal separator, a coolant flow field for allowing a coolant to flow being formed in each space between the power generation units, wherein:

• a same reactant gas flows through the first and third reactant gas flow fields,
• ridges and flow grooves are formed in the first reactant gas flow field and in the third reactant gas flow field for distributing the same reactant gas to the power generation surfaces, the ridges and flow grooves being formed by corrugating the first metal separator and the second metal separator, wherein the ridges of the first and third reactant gas flow fields extend in a first direction and exist between the flow grooves of the first and third reactant gas flow fields, and the flow grooves of the first and third reactant gas flow fields extend in a second direction opposed to the first direction,
• a number of flow grooves in the first reactant gas flow field is different from a number of flow grooves in the third reactant gas flow field, wherein when the fuel cell is assembled, a groove of the first reactant gas flow field substantially aligns with a ridge of the third reactant gas flow field,
• the first reactant gas flow field and the third reactant gas flow field have a same length, and
• the flow grooves in the first reactant gas flow field and the flow grooves in the third reactant gas flow field have a same depth.