When an electric power generation demand occurs for a fuel cell stack 1, an electric power stored in a battery 8 is commenced to be supplied to a load (step S3) followed by setting a period (step S2), in which a sensor signal from a fuel cell system 10 is held in a preceding value, in a subtraction timer, and when discrimination is made (step S5, step S6) that the fuel cell system 10 is started up and each pressure is stabilized from a sensor signal from the fuel cell system 10, the electric power generated in the fuel cell stack 1 is commenced to be supplied to the load (step S8) followed by setting the period, in which the sensor signal from the fuel cell system 10 is held in the preceding value, in the subtraction timer (step S7).