A fuel cell system includes a stack case storing a fuel cell stack, and an auxiliary device case containing a fuel gas system device and an oxygen-containing gas system device. The auxiliary device case covers the fuel gas system device in a manner to protect the fuel gas system device against the external load, and includes a first case member provided with a mount fixed to a vehicle body frame, and a second case member made of material having specific gravity smaller than that of the first case member, and covers at least the oxygen-containing gas system device.