The method controls a differential pressure of a plate reformer installed in a fuel cell power generation system. In the power generation system, a raw material gas is reformed to a fuel gas in the plate reformer, the fuel gas is fed to an anode of a fuel cell, an anode exhaust gas from the anode is fed to a combustion chamber of the plate reformer a combustion exhaust gas from the combustion chamber is fed to a cathode of the fuel cell together with an air, and part of the air is fed to the combustion chamber. The method includes the steps of placing the plate reformer in a pressure vessel, maintaining a pressure of a cathode exhaust gas to a constant value, feeding part of the air to be fed to the cathode and all of the anode exhaust gas into the combustion chamber to make a pressure of the air and that of the anode exhaust gas substantially the same so as to make a pressure of the reforming chamber and that of the combustion chamber substantially the same, and forming an opening on a line which feeds the air into the combustion chamber to allow the air to enter the pressure vessel so as to make the pressure of the reforming chamber, that of the combustion chamber and that of the pressure vessel substantially the same.