In recent years, fuel cell for generating electricity by an electrochemical reaction between hydrogen and oxygen has been focused on as an energy source. The fuel cell is configured with a hydrogen electrode and an oxygen electrode sandwiching an electrolyte. When hydrogen-rich fuel gas is supplied to the hydrogen electrode and oxidizing gas such as air is supplied to the oxygen electrode, the hydrogen and oxygen in these gases react to form water while generating electricity. This reaction mainly occurs at the oxygen electrode. When a large amount of water is formed by the reaction, there is a case to be impossible to discharge the formed water and the formed water causes clogging of oxidizing gas piping depending on the fuel cell operating condition. This phenomenon called flooding causes interference of oxidizing gas supply to the oxygen electrode. By the interference, the performance of the electricity generation becomes lower and an adverse effect such as shortening of the fuel cell lifetime may occur. The flooding occurs easily when the operating condition for which the oxidizing gas flow rate is relatively small, such a condition as generating electricity at a low current density.
JP63-110558A discloses a technique for suppressing flooding by increasing the flow rate of oxidizing gas intermittently.
To increase the flow rate of oxidizing gas, it is necessary to increase the pump power of a pump for supplying the oxidizing gas. Increase of the pump power may cause a decrease in energy efficiency of fuel cells. Also the increase in the flow rate may be delay after the increase of the pump power, so there is the problem that the responsiveness is not enough for suppression of flooding by increasing the flow rate of the oxidizing gas. Due to this kind of low responsiveness, with the conventional art, it is necessary to increase the oxidizing gas before the flooding occurred, and there is also the problem of decreasing the energy efficiency. The present invention is created considering these problems, and its purpose is to provide a technology capable of avoiding flooding while suppressing a decrease in energy efficiency.