In a fuel cell having a separator in which main cooling water channels are formed, a separator for another unit cell stacked on the cooling water channel formation surface side of the separator, and a second sealing member interposed between the separators and to seal a cooling medium flowing in the main cooling water channels, an outer peripheral rib for regulating the flow of cooling water to the second sealing member side is provided inside relative to the second sealing member in the separator surface direction in order to improve the efficiency of cooling with the cooling medium.