PURPOSE: To insure temperature adjustment in a wide load range, and reduce exhaustion of nitrogen oxide.

CONSTITUTION: A mixture of gas and air supplied through a primary mixture supply conduit 2 is separated through a secondary mixture chamber 3 and a primary mixture chamber 6. A flow rate of the mixture is relatively low with a low load, and combustion of a primary mixture flow happens in a primary exhaust outlet 7. When the load is increased, the flow rate of the mixture is increased beyond a combustion rate of the mixture to blow out a flame, but the primary mixture flow is preheated owing to existence of the secondary mixture flow flowing from the secondary mixture chamber 3, whereby a combustion rate of the primary mixture flow is increased to permit a stable flame to appear. A front part of the flame is configured into a fan shape owing to a high flow rate of the primary mixture flow in the primary exhaust outlet 7 and combustion is spreaded over a large area, so that combustion temperature is lowered and hence a composition of nitrogen oxide is sharply reduced.