PURPOSE: To reduce NOx generated in exhaust gas from a boiler, especially in a medium and/or small capacity boiler, by utilizing an ignition burner which is able to adjust automatically the diection of fuel injection and serves also as a subburner.

CONSTITUTION: A burner 7a and 7b are arranged in two stairs. The lower burner 7b is ignited by a fixed ignition burner 2 connected thereto, and the upper burner 7a is provided with an ignition burner which is able to adjust the direction of fuel injection. When the burner is ignited, the ignition by fuel gas occurs at the position 11 shown in the drawing, and as the burner reaches regular load the ignition burner is pivoted to the position 11' from the position 11. Then fuel oil is supplied thereto, and the burner is utilized as a subburner used in low air ratio. Consequently, said device serves both as an ignition burner and a subburner, and also can reduce effectively NOx generated in the medium and/or small capacity boilers.