A fuel cell stack is an energy conversion system for converting chemical energy to electric energy through an electrochemical reaction caused by supplying a fuel gas and an oxidant gas to a membrane-electrode assembly. In particular, a solid polymer electrolyte fuel cell stack in which a solid polymer membrane is used as an electrolyte is low in cost, easy to be reduced in size, and high in output density, and therefore is expected to be used as a vehicle-mounted power source.
Regarding a fuel cell system equipped with such a fuel cell stack, for example, cited reference 1 below proposes a method of enhancing the operation efficiency of a system by setting, with the use of a DC/DC converter, an output of a fuel cell to account for 65% to 80% of the entire output.    Patent Document 1: JP2002-118979 A