A first separator of a unit cell and a second separator of another unit cell and are disposed adjacent to each other in a stacked assembly. Crest surfaces of straight sections of first hollow ridges of the first separator are in contact with crest surfaces of second hollow ridges of the second separator, and crest surfaces of bent sections of the first hollow ridges are spaced from crest surfaces of the second hollow ridges. The spaced crest surfaces allow first troughs of the first separator and second troughs of the second separator to communicate with each other, providing communication passages between the first separator and the second separator. Cooling water is passed through the communication passages.