An oxygen-containing gas supply passage and an oxygen-containing gas discharge passage extend through a set of diagonal positions of a first metal separator, and a fuel gas supply passage and a fuel gas discharge passage extend through the other set of diagonal positions of the first metal separator. A fuel gas flow field is connected to the fuel gas supply passage through an inlet buffer at an upper position, and connected to the fuel gas discharge passage through an outlet buffer at a lower position. The inlet buffer includes a first inlet buffer area adjacent to the fuel gas supply passage and a second inlet buffer area adjacent to the fuel gas flow field. Grooves of the first inlet buffer area are deeper than grooves of the second inlet buffer area in the stacking direction.