At least one separator of the two separators is formed of a press-formed plate having recesses and protrusions. Among portions of the recesses and protrusions, a portion coming toward the MEGA plate is designated as recessed portion, and a portion going apart from the MEGA plate is designated as protruded portion. The one separator has a first recessed portion bonded to the frame member, a first protruded portion contiguous to the first recessed portion, and a second recessed portion formed on one side of the first protruded portion opposed to the first recessed portion. The fuel cell stack is capable to take a tightened state in which a tightening load is imparted to the plurality of unit cells by the tightening member, and a non-tightened state in which no tightening load is imparted. The unit cells are so configured that the second recessed portion is in contact with the frame member in the tightened state, and the second recessed portion is out of contact with the frame member in the non-tightened state.