Object: To provide a recovery boiler for recovering soda component from pulp spent liquor, which is able to prevent carry-over of unburnt char and deformation of char bed configuration and to attain a stable combustion with low NOx generation. Solving means: By regulating combustion air supply, and feeding inert gas along furnace side wall around char bed as the case may be, there are formed a combustion zone of reduction atmospheric field where air ratio in surroundings of char bed is 0.8 or less, a combustion zone of reduction atmospheric field where air ratio is 1.0 or less and unburnt components exist (including a case to form reduction atmospheric field where air ratio is 1.0 or less and unburnt components exist with said two combustion zones being combined together) and a combustion zone where combustion completes.
