A separator includes an electrically conductive flat plate and two electrically conductive channel plates superposed on each other with the electrically conductive flat plate interposed therebetween. The separator includes a plurality of manifolds for passing a reaction fluid to an adjacent cell are provided in the electrically conductive flat plate, and a plurality of zigzag or straight through-channels for permitting the flowing of the reaction fluid from the manifolds are formed in each of the two channel plates. The separator includes the through-channels in any one of the two electrically conductive channel plates disposed so that they are partially overlapped with the manifolds in the electrically conductive flat plate, and the through-channels in the other electrically conductive channel plate are formed to be partially larger than the contour of the electrically conductive flat plate.