FUEL CELL SYSTEM

A fuel cell system comprising: a fuel cell stack including: a stacked body in which unit cells are stacked, the unit cells including first and second unit cells; a reactant gas flow path; and a cooling water flow path; a pump that supplies cooling water to the cooling water flow path; a supply device that supplies reactant gas to the reactant gas flow path; and a control device configured to include: a bubble detection portion configured to detect an accumulation of bubbles in the cooling water flow path; and a cause determination portion configured to determine whether or not the accumulation of bubbles is caused by leakage of the reactant gas from the reactant gas flow path, when the bubble detection portion detects the accumulation of bubbles.