PURPOSE: To increase energy-saving effect of a small capacity of boiler and reduce heat loss due to sensible heat of exhaust gas by controlling stepwisely control signal voltage of each flow adjusting mean which is selected by switching mean that makes specified resistance value in accordance with signal from each detector.

CONSTITUTION: First stage throttling operation for a valve 30 is made by signal voltage E due to resistance 35W38. Furthermore, when a signal is sent off from a detector 14, a contact 45 is ON and applied voltage to a resistance 35 is cut off and signal voltage E drops to the voltage caused by combined resistance of resistances 36, 37 and then second-stage throttling operation for the valve 30 is made. When signal is dispatched again from the detector 14 in spite of this condition, next contact 46 is ON and applied voltage to a resistance 36 is cut off and third stage throttling operation is performed and further, as signal is sent off from the detector 14, next contact is ON and fourth-stage throttling operation is made by voltage E2 due to a resistance 37. Thereby, a device has simple construction and fuel and air amounts are controlled with fine accuracy.