In a burner of construction having a primary nozzle (4), a secondary nozzle (8) and a tertiary nozzle (9), a partition wall (1, 2) partitioning the secondary nozzle (8) and the tertiary nozzle (9) and having a flow path change member (3) provided thereon, the partition wall (2) is formed so as to be movable in parallel to the burner axis to control jetting speeds and flow rates of secondary air and tertiary air, whereby it is possible to cool the burner constituent members while reducing NOx. The partition wall (1, 2) is composed of a fixed wall (1) and a movable wall (2). The burner comprises a bypass passage (16, 19) through which tertiary air in the tertiary nozzle (9) bypasses the tertiary nozzle to flow into the secondary nozzle (8) or the primary nozzle (4).