After a fuel cell power plant using a fuel cell stack stops operating, in the fuel cell stack, air existing in a cathode may permeate an electrolyte membrane in the direction of an anode, and if a minute amount of fuel gas is left in the anode, the air that has passed into the anode may react with the fuel gas remaining therein to form a local cell in the anode. 
This local cell forming phenomenon in the anode is expressed in FIG.  11.  Herein, the main component of the fuel gas is hydrogen (H2) and the oxidant gas is oxygen (O2) in the air. 
In a part of the anode 2, hydrogen (H2) is separated into a hydrogen ion (H+) and an electron (e−), as expressed by the chemical formula (1) below. H2→2H++2e−  (1)
The hydrogen ion (H+) permeates the electrolyte membrane and reaches the cathode.  The electron (e−) moves to another part of the anode 2, and reacts with the hydrogen ion (H+) and oxygen (O2) to form water (H2O), as expressed by the chemical formula (2) below. O2+4H++4e−→2H2O  (2)
The hydrogen ion (H+) used for this reaction is provided from the cathode 3 through the electrolyte membrane 1A.
In the cathode 3, oxygen (O2) reacts with the hydrogen ion (H+) provided from the anode and the electron (e−) provided from the other part of the cathode 3 to form water as expressed by the chemical formula (3) below. O2+4H++4e−→2H2O  (3)
In the other part of the cathode, the following reactions occur as expressed by the chemical formulae (4), (5) below. C+2H2O→CO2+4H++4e−  (4)2H2O→O2+4H++4e−  (5)
The reactions expressed by the chemical formulae (1) and (3) are normal reactions in a fuel cell.  The reactions expressed by the chemical formulae (2), (4), (5) are not.  These reactions are required for consuming an electron (e−) that is produced in the other part of the anode 2 or for producing an electron (e−) that is consumed in the other part of the cathode 3. 
As a result, the electron (e−) moves across the anode 2 and across the cathode 3, and a power current is generated in the anode 2 and the cathode 3 respectively. 
Carbon (C) is used in the reaction expressed by the chemical formula (4).  This carbon is obtained from a carbon material which forms a catalyst layer of the cathode 3, and as a result this reaction causes corrosion of the carbon layer of the cathode 3.  This corrosion of the carbon layer of the cathode 3 is known as local cell corrosion. 
In order to prevent deterioration of a membrane-electrode assembly (MEA) including local cell corrosion, JP2004-139950A, published by Japan Patent Office in 2004, proposes connecting the fuel gas passage to the oxidant gas passage and hermetically sealing these passages while electrically connecting an electrical load to the anode and cathode so as to cause the fuel gas and oxidant gas remaining in the fuel cell stack to be consumed through power generation, thereby suppressing the MEA from deteriorating after an operation stop of a fuel cell stack.