PURPOSE: To control combustion with high accuracy, and to conserve energy and prevent environmental pollution by a method wherein O2 concentration obtained in a region containing no branching point for exhaust gas is used, the condition of the flow of exhaust gas in the furnace is grasped, the furnace is separated in response to the condition, and the O2 concentration of exhaust gas is controlled in the multi-belt type combustion furnace in which exhaust gas is branched in two directions in the furnace.

CONSTITUTION: A rotary furnace 1 consists of eight combustion chambers, such as a first zone 1a with a plurality of axial-flow burners 21, a second zone 1b with an axial-flow burner 22 and a eighth zone 1h with ceiling burners 281, 282 and the like, and a rotary hearth 1n is turned and moved in the furnace. A material to be heated is charged into the rotary furnace 1 from a charge port, the rotary hearth 1n is turned and moved in the direction of the arrow defined by an outline, the temperature of the material to be heated is elevated gradually, and the material is extracted from the eighth zone 1h. O2 concentration in exhaust gas is measured by O2 concentration analyzers 4, 4β, 4γ each connected to gas sampling devices 3α, 3β, 3γ and signals as the results of analysis are transmitted to an arithmetic unit 5. The arithmetic unit 5 computes air ratios through which the material must be operated in each zone on the basis of inputted data, and inputs the air-ratio operating output signals to a combustion controller 6, and the combustion controller 6 adjusts the air ratios of each zone in accordance with the signals.