PURPOSE: To reduce NOx by providing a combustor nozzle with a first injection nozzle for supplying oxidation furnace gas and a second injection nozzle provided round the first injection nozzle for supplying gasification furnace gas and an air injection nozzle on the outside of the second injection nozzle.

CONSTITUTION: The combustion nozzle of a combustor 7 has an oxidation furnace gas injection nozzle 15 in the center, and round the nozzle 15 a plurality of gasification furnace gas injection nozzles 16 are provided to reduce the NOx contained much in the combustion gas. And an oxidation reaction occurs by the air from an injection nozzle 17 of combustion air provided in the outmost circumference and as a whole the NOx is reduced. The NOx contained in the oxidation furnace gas is, therefore, reduced when it comes out of the combustion nozzle of the combustor 7.