PURPOSE: To obtain effective reduction of NOx contained in waste gas being generated from boiler, by not performing recirculation of self waste gas by burner along, but performing the same operation in the system cooperating with the function of water tubes.

CONSTITUTION: Inbetween the upper drum 3 and the lower drum 4 in the boiler furnace, (a) water wall made by a group of water wall tube 5 using tubes, and (b) venturi made by water cooling pipes having venturi blades made of a group of venturi blade tube 6, are constituted. By the function of burner 1 attached on the wind box 2, mixture of fuel and air for combustion is injected at rapid speed, and the waste gas generated within the furnace is sucked into the burner flame by attracting the waste gas to the group of water wall pipes 5 and watr cooling pipe formed by the group of venturi blade pipe 6. That is, waste gas being cooled by watercooling pipes is recirculated by the burner 1 itself and is mixed with the combustion gas.