A stable and high reliability fuel cell electricity-generating device capable of generating electricity even in the case of sudden drop of load power. A fuel cell generating electric power from a fuel and an oxidizer, a fuel processor producing fuel to be supplied into the fuel cell from an electricity-generating material, a combustion device combusting a residual fuel gas unconsumed in the fuel cell to raise the temperature of the fuel processor, and an electric power generation instructing means of determining the electric power generated by the fuel cell, wherein when the electric power generation instructing means decreases the electric power generated by the fuel cell depending on the decrease of load power to be supplied, the rate at which the generated electric power is decreased is made different depending on the change of the temperature of the fuel processor.