Experiments were conducted to improve thermal efficiency of a waste heat boiler fitted to a commercial fluidized bed incinerator being supplied with municipal solid waste (MSW). As a result, it was found that the control method, which is a operation of stopping fluidization instantaneously for the purpose of preventing excess steam generation, is effective. Furthermore, the daily on-off operation of a fluidized bed incinerator, which is the operation of maintaining the operational steam pressure using the heat accumulation in sand without feeding MSW at night and performing maximum combustion in the daytime, was confirmed to be effective for producing excess electric power.