Mount structure of a fuel cell stack holds the fuel cell stack in a motor room of a vehicle by side mounts and a rear mount. The fuel cell stack includes a plurality of power generation cells and a stack case. A window frame having a rear window is provided in a rear surface of the stack case. A device of the fuel cell system can be inserted into the rear window. The rear mount includes a rear mount bracket attached to a window frame. The rear mount bracket is positioned is below a vehicle body frame and a rear window.