A fuel cell system includes a fuel cell stack. The fuel cell stack includes a plurality of fuel cells stacked together in a stacking direction, and end plates provided at opposite ends of the fuel cells in the stacking direction. A fluid unit is provided on one side of the fuel cell stack. The fluid unit includes a heat exchanger for heating an oxygen-containing gas to be supplied to the fuel cell stack, and a reformer for reforming a fuel to produce a fuel gas.