A fuel cell stack formed by stacking two or more fuel cell layers each constituted of one or more unit cell and a fuel cell system including the same are provided. Any two fuel cell layers adjacent to each other each have one or more gap region. At least a part of the gap region in one fuel cell layer of any two fuel cell layers adjacent to each other is in contact with a unit cell constituting the other fuel cell layer. The gap region in one fuel cell layer and the gap region in the other fuel cell layer communicate with each other. The fuel cell stack is excellent in fuel or oxidizing agent supply performance and it realizes high power density.