A fluidized bed incinerator for municipal solid wastes (MSW) has excellent characteristics with a high combustion rate. But they sometimes burn incompletely when the wastes supplied to the bed change greatly in feed rate and properties. As a countermeasure for this, a fluidization at the center of the bed, where wastes were supplied, was kept rather weak and that at the both side was made strong and weak alternately. Then the heat transfer coefficient to the wastes was decreased and pyrolysis was delayed. At the same time by blowing secondary air to the freeboard at the optimum position and flow rate, a stable combustion could be realized. At the test of the actual MSW fluidized bed incinerator, it was confirmed that the frequency of high CO emisson spike was decreased and that visual smoke was suppressed than the conventional fluidized bed incinerator.