Fuel cell system and control method of fuel cell system

A method of controlling a fuel cell system including a fuel cell to be supplied with an anode gas and a cathode gas for generating electric power in accordance with an external load, a pressure regulating valve for controlling a pressure of the anode gas supplied to the fuel cell, and a purge valve configured to change an opening area thereof at least on two stages, for controlling a discharge amount of an anode off-gas discharged from the fuel cell, wherein:

• the purge valve includes a first purge valve provided on a first purge passage for discharging the anode off-gas to an outside air, and a second purge valve provided on a second purge passage that branches from the first purge passage and is connected to a cathode gas supply passage for supplying the fuel cell with the cathode gas,
• the method comprising the steps of:
• controlling the pressure regulating valve so that the pressure of the anode gas in the fuel cell when the external load is high becomes higher than when the external load is low, and so that the pressure of the anode gas is periodically increased and decreased at a predetermined load;
• and
• increasing an opening area of the purge valve used during a descending transition operation relative to the opening area used during other operations by opening the second purge valve during the descending transition operation.