Abstract: A fuel cell system in which water can be accurately supplied in accordance with the flow rate of fuel gas supplied to the reformer to allow efficient reforming without using a flow meter measuring the flow rate of water. In the fuel cell system in which fuel gas is supplied to a reformer where steam reforming is carried out to convert the fuel gas to hydrogen rich gas, and the hydrogen rich gas is supplied to a fuel cell unit to generate electricity, the pump that supplies water to the reformer is a plunger pump in which the rate of discharge of water is proportional to the rate of rotation of the motor that drives the pump. Also the fuel cell system includes a gas flow rate sensor to measure the flow rate of fuel gas supplied to the reformer, and a controller that controls the rate of rotation of the motor in accordance with the value of flow rate measured by the gas flow rate sensor.