A stack for a fuel cell system includes an electricity generating unit,
end plates, a restraint portion, and an element for reducing stress. The
electricity generating includes a membrane-electrode assembly (MEA) and
separators respectively disposed on opposite surfaces of the MEA. The end
plates are respectively disposed at opposite sides of the electricity
generating unit. The restraint portion is joined with the end plates to
fasten the end plates and the electricity generating unit together. The
restraint portion includes a restraint rod which is disposed in the end
plates, and a nut which is screw-joined with the restraint rod so that
the restraint rod is fastened to the end plates. The element for reducing
stress concentration is coupled to each of the end plates to reduce (or
minimize or prevent) a stress force generated by the nut from
concentrating at a portion where the restraint portion is disposed.