PROBLEM TO BE SOLVED: To appropriately perform ventilation from a frame in a fuel cell system even when reducing a drive amount for a ventilation fan in order to determine the existence of abnormality of a flow rate detector for detecting a flow rate of fluid generated accompanying the ventilation.SOLUTION: A fuel cell system comprises: a frame 10a for accommodating a fuel cell 34; a ventilation fan 42 for performing ventilation of the inside of the frame 10a; a flow rate detector 60 for detecting a flow rate (Q) of fluid generated accompanying the ventilation; and a controller 15. The ventilation fan 42 is disposed in an exhaust part Ex in which fluid discharged from the frame 10a flows in performing the ventilation. The controller 15 comprises a first abnormality existence determination unit for determining the existence of abnormality of the flow rate detector 60 by checking an output signal of the flow rate detector 60 by making a duty ratio (D) of the ventilation fan 42 be a checking duty ratio (Dc) that is larger than zero and equal to or smaller than a changeover duty ratio (Dk) corresponding to a changeover flow rate (Qk) at which the output signal of the flow rate detector 60 changes over.
