Upon determining the temperature of and pressure in a flow path connecting between a fuel cell and a fuel storage container for supplying fuel to the fuel cell in order to detect a residual capacity of a fuel cell battery, the temperature is determined from those measured in electronic equipment connected to the fuel cell battery. Therefore, there can be provided a residual capacity detection method and a residual capacity detection system for a fuel cell battery, which can give a user accurate information on battery residual capacity even if there is no temperature detection part in a fuel cell system. The present invention can also be understood as a residual capacity detection system for a fuel cell battery.