A fuel cell stack 5 adapted for an electrochemical reaction of a fuel gas to generate an electric power is provided with a working point measuring section 51 for measuring a generation voltage and a generation current of the fuel cell stack 5, and connected to a controller 7 including an output characteristic map memory 72 for storing output characteristics of the fuel cell stack 5, and a potential output power estimator 71 for estimating a potential output power of the fuel cell stack 5 based on a combination of a current generation voltage VO and a current generation current IO measured by the working point measuring section 51 and pieces of information stored in the output characteristic map memory 72.