The accuracy of detecting gas leakage in a fuel battery system is improved. A fuel battery system includes a fuel battery to which a reactive gas is supplied to generate power, and a gas passage (a fuel gas supply path and a fuel gas circulation path) connected to this fuel battery, this gas passage is provided with a plurality of adjoining closed spaces, and the system includes a detecting unit (a control section) to detect gas leakage in one closed space in a state in which at least a pressure of another closed space adjoining the one closed space as a gas leakage detection target on a downstream side is lowered.