A power generation unit of a fuel cell stack includes a first metal separator, a first membrane electrode assembly, a second metal separator, a second membrane electrode assembly, and a third metal separator. A first oxygen-containing gas flow field includes a plurality of wavy flow grooves. An outlet merging area is provided at the end of the wavy flow grooves on the outlet side. The outlet merging area is connected to a plurality of straight connection flow grooves. The groove depth of the straight connection flow grooves is smaller than the groove depth of the wavy flow grooves.