A fuel cell system having a fuel cell, a reactant gas pipe for supplying a reactant gas to the fuel cell, and an injector for driving a valve body at a predetermined drive cycle by an electromagnetic driving force to separate the valve body from a valve seat, regulating conditions of the gas on the upstream side in the reactant gas pipe and supplying the gas to the downstream side. A gas element component responding to the physical quantity of the reactant gas circulating through the reactant gas pipe is integrally provided in the injector so as to come close to the injector.