A fuel cell stack comprising a second metal separator set to have an external dimension larger than a first metal separator, wherein the second metal separator comprises, formed integrally, a first seal member in contact with the peripheral edge of a first electrolyte membrane/electrode structure, a second seal member in contact with the peripheral edge of the first metal separator, and a third seal member in contact with the peripheral edge of an adjoining fourth metal separator. Since the first seal member, the second seal member and the third seal member are integrally formed on one surface of the second separator or one surface of the first separator, a seal-forming step can be carried out at one effort, simply and economically. In addition, use of a triple seal structure containing the first through the third seal members can favorably improve the sealing feature of reaction gas and minimize reaction gas leakage.