The invention has its object to arbitrarily adjust an amount of particles to be circulated without changing a flow rate of a gasification agent to thereby enhance gasification efficiency in a fluidized bed gasification furnace.The fluidized bed gasification furnace 107 comprises first and second chambers 113 and 114 in communication with each other in a fluidized bed 105. The hot particles 102 separated in the separator 104 and raw material M are introduced into the first chamber 113. The particles 102 introduced from the first chamber 113 through interior in the fluidized bed 105 to the second chamber 114 are supplied in an overflow manner to the fluidized bed combustion furnace 100. A first pressure controller 121 is provided to control the resultant gas induction means 116 such that the pressure in the first chamber 113 is kept to preset pressure 120; and a second pressure controller 124 is provided to control the exhaust gas induction means 118 such that difference between pressure in the first and second chambers 113 and 114 is equal to the preset differential pressure 123, so that the fluidized bed 105 in the first chamber 113 is controlled in height to control an amount of particles 102 to be circulated.