A first seal member is formed integrally on both surfaces of a first metal plate. The first seal member is integrally formed on a cooling surface of the first metal plate, except a region corresponding to a reaction surface facing an electrode reaction surface, and except regions of inlet buffers and outlet buffers. The first seal member has an expansion. The position of an end surface of the expansion substantially matches the position of a wall surface of the outermost groove of a coolant flow field to prevent the flow of a coolant around the electrode reaction surface.