A fluidized bed combustor wherein a combustion chamber and a regeneration chamber are both contained in a single hollow body is provided. These two chambers are formed by vertically partitioning the body by a partition wall, which has an upper opening and a lower one, and also each have a perforated plate at the bottom part thereof on which a heat transfer medium containing a desulfurizing agent is placed and fluidized. The desulfurizing agent is transferred from the combustion chamber through the lower opening to the regeneration chamber and circulated by overflow through the upper opening again to the combustion chamber. Various modifications of the above-mentioned fundamental embodiment are proposed. Combustion and desulfurization are automatically and effectively carried out in a single apparatus to give a high percentage desulfurization, make the apparatus compact, and reduce the initial cost and the running cost thereof.