When the output of a fuel cell starts to decrease, in consideration of the increase in the power generation efficiency, an instruction for the output of the fuel cell undergoes a smoothing process to gradually decrease the output, and excessive electric power generated during a period in which the power generation efficiency is high is used for charging a battery. While the output of the fuel cell is increasing, the power generation efficiency of the fuel cell is low. Thus, assisting electrical energy from the battery is increased. In this manner, the power generation output when the power generation efficiency of the fuel cell is low is reduced from a high power generation output to a low power generation output to achieve improvement in the average power generation efficiency.