A fuel cell system includes a stack case containing a fuel cell stack therein. The stack case includes first and second end plates, first and second side plates, top and bottom cover plates. A top sealing member has a substantially rectangular frame shape and disposed between the top cover plate and each of the first end plate, the second end plate, the first side plate and the second side plate. A bottom sealing member has a substantially rectangular frame shape and disposed between the bottom cover plate and each of the first end plate, the second end plate, the first side plate and the second side plate. At least one of the top sealing member and the bottom sealing member including, at at least one corner of the substantially rectangular frame shape, an extension portion to adjust a peripheral length of the substantially rectangular frame shape.