Each of power generation units of a fuel cell according to the present invention is formed by stacking a first metal separator, a first membrane electrode assembly, a second metal separator, a second membrane electrode assembly, and a third metal separator together. The power generation unit has therein a first oxygen-containing gas flow field, a first fuel gas flow field, a second oxygen-containing gas flow field, and a second fuel gas flow field. The number of flow grooves in the first oxygen-containing gas flow field is different from that of flow grooves in the second oxygen-containing gas flow field, and the number of flow grooves in the first fuel gas flow field is different from that of flow grooves in the second fuel gas flow field.