PURPOSE: To obtain an effective operation of the burner under a constant extra air coefficient over the entire load range, particularly in a low load range by a structure wherein a sleeve provided in a burner casing is divided into two parts, each connected to an independent air passage and the air passage is equipped with a damper.

CONSTITUTION: The sleeve provided in the burner casing 1 is divided into two parts, a primary sleeve 2 and a secondary sleeve 3; the primary sleeve 2 having a primary throat 5 and inserted therethrough with a sprayer body 10 which is provided with a primary swirler 11 and an atomizer 12 positioned within the primary throat 5, and the secondary sleeve 3 inserted therethrough with the primary sleeve 2. These primary and secondary sleeves are connected respectively to independent air passages a and b and equipped respectively with a primary damper 8 and secondary dampers 9a and 9b. Within a low load range of combustion in which a boiler load is less than 50%, a primary combustion air is supplied into the primary throat 5 via the primary damper 8, air passage a and primary sleeve 2. In this instance, flowing speed of the air is maintained by the primary swirler 11 and primary throat 5. The fuel is thus effectively mixed with the primary air to be burned with a certain amount of extra air even at low load.