A fuel cell stack includes a stacked body including unit cells stacked. The stacked body is provided with reactant gas flow paths through which reactant gases flow, and a cooling water flow path through which cooling water flows. When bubbles accumulate in the cooling water flow path, the cooling efficiency of the fuel cell stack is lowered, which may degrade power generation efficiency thereof. For solving this problem, in Japanese Unexamined Patent Application Publication No. 2014-86156, the bubbles are discharged from the cooling water flow path by changing the rotating speed of the pump that supplies cooling water to the cooling water flow path.
As described above, when the bubbles accumulate in the cooling water flow path, it is useful to take an appropriate action depending on a cause of the accumulation of bubbles. Conventionally, however, the cause has not been determined.