Abstract: In a petroleum residuum burning boiler including: a high-temperature reduction combustion chamber to which petroleum residuum fuel and primary combustion air are supplied and in which combustion is performed at a temperature of 1,300° C. or more and an air ratio of less than one; and a low-temperature oxidation combustion chamber which is connected to the high-temperature reduction combustion chamber and in which combustion is performed at a temperature of less than 1,300° C. and an air ratio of one or more, an assist gas is supplied to the high-temperature reduction combustion chamber, and unburned carbon of a combustion gas of the petroleum residuum fuel is gasified by a water gas reaction by using steam, generated by combustion of the assist gas, as a gasifying agent.