To provide a recovery boiler composed so as to remove K and Cl accumulated in a black liquor line and to make solvable problems such as the corrosion of the boiler and sticking of ash to a superheater, etc., in the boiler.

The recovery boiler 15 is capable of feeding a part of ash recovered from a waste gas from the recovery boiler 15 with an ash recovery device 16 to a potassium removing and desalting device 17, adding water to the ash in the potassium removing and desalting device 17, then separating crystalline Na2SO4 from the liquid containing the K and Cl and feeding the resultant Na2SO4 together with the ash from the ash recovery device 16 to a mixing tank 14. Thereby, the K and Cl are removed from a black liquor line of the recovery boiler 15.