1. An anomaly judgment device of a fuel cell system, comprising: a first anomaly judgment device to judge a gas leak of the system that includes a first judgment device that judges whether there is an anomaly in cases where a fuel gas density detected by a detection device is equal to or more than a predetermined value; a second anomaly judgment device to judge a gas leak of the system that includes a second judgment device that judges that there is an anomaly in cases where the change in pressure in fuel gas piping sealed by a sealing device is equal to or more than a predetermined value; and a judgment device to implement an anomaly judgment using the second anomaly judgment device in cases where it is judged by the first anomaly judgment device that there is an anomaly and to judge that a system anomaly exists in cases where it is also judged by the second anomaly judgment device that there is an anomaly, and wherein the judgment device activates the second anomaly judgment device in cases where it is judged by the first judgment device that there is an anomaly.