A cell unit of a fuel cell includes a second separator. A first oxygen-containing gas flow field is formed on a surface of the second separator. An inlet buffer is connected to an inlet of the oxygen-containing gas flow field, and an outlet buffer is connected to an outlet of the first oxygen-containing gas flow field. The inlet buffer includes a first inlet buffer area having a deep groove and a second inlet buffer area, and the outlet buffer includes a first outlet buffer area having a deep groove and a second outlet buffer area. The first inlet buffer area and the first outlet buffer area have different surface areas.