An abnormality detecting device of a fuel cell system according to the invention includes a hydrogen off-gas circulation passage for making hydrogen off-gas discharged from a fuel cell flow back to an anode; a discharge passage for discharging part of the hydrogen off-gas, which is circulated through the hydrogen off-gas circulation passage, from the hydrogen off-gas circulation passage; a hydrogen discharge valve provided in the discharge passage; abnormality determining means for determining whether an abnormality has occurred in opening/closing of the hydrogen discharge valve and gas state quantity detecting means for detecting a gas state quantity of the hydrogen off-gas, the gas state quantity detecting means being provided in the discharge passage at a position downstream from the hydrogen discharge valve. The abnormality determining means determines whether an abnormality has occurred in opening/closing of the hydrogen discharge valve based on the gas state quantity of the hydrogen off-gas.