PURPOSE: To restrict NOx emission to be low without permitting gas and air to be mixed together in an early time, by employing a gaseous fuel for a fuel supplied through an injector, and adjusting a velocity of the fuel invading into an inner chamber of a burner to be the same as that of an air stream or lower.

CONSTITUTION: A burner comprises half partial conical bodies 2, 3 disposed, displaced vertically each other, and tangential invasion slits 2b, 3b are formed by displacing central axes 2a, 3a of the partial-conical bodies 2, 3 each other, the slits having a predetermined slit width S on both sides of the burner. Invasion passages 23, 24 include injectors 12, 13, which are adapted such that a gaseous fuel 4 flows into gas injector passages 12b, 13b after passage through a plurality of gas holes 14 from gas supply passages 12a, 13a. An invasion speed of the fuel 4 supplied through the injectors 12, 13 with which speed the fuel invades into an inner chamber 17 is adjusted to be same or lower as that of an air stream 7 mixed with the fuel 4 in the range of the invasion slits 2b, 3b.