A failure diagnostic device for accurately determining a failure of a discharge valve. A failure diagnostic device (50) serves to perform failure diagnostic of a discharge valve (62) installed in a discharge flow channel (32) of an anode discharge gas that is discharged from a fuel cell stack (20). The device comprises a detection unit (70) for detecting a status quantity of the anode discharge gas between a discharge port (27) for the anode discharge gas of the fuel cells tack (2) and the discharge valve (62) and a determination unit (51) for determining a failure of the discharge valve (62) based on the status quantity of the anode discharge gas detected by the detection unit (70) and a failure determination quantity corresponding to the operating state of the fuel cell stack (20).