The present invention provides a fuel cell system that has a fuel cell portion including one or more unit cells, a fuel tank for storing fuel used in the unit cells, and a control portion for controlling the fuel cell portion. The number of unit cells is optimized so that a total amount of energy, which is outputted from the fuel cell system as a function of the number of unit cells or of an electrode area, shows a maximum value or a nearly maximum value. Thus, the amount of energy may be maximized or made to be equal to or more than a predetermined value. Accordingly, target devices to be supplied with power from the fuel cell system may be used for a longer time.