PURPOSE: To increase the stability of flame of a burner upon reducing the load of the burner and contrive the expanding of turn down.

CONSTITUTION: The opening degree of a resistor vane for a burner 3 is controlled by a method wherein fire-retardant fuel 10 is injected out of a burner gun 4 to burn from an output command 17 and, simultaneously, the opening degree of a resistor vane 18 is adjusted from the output command 17 whereby secondary air 7 is supplied while whirling it so as to surround flame F formed by the burner gun 4. When the flickering level of of the flame F is detected, the level of a burner load signal 38 is low and the flickering level signals 36a, 36b of the flame F are lowered to a lower limit set value, the opening degree of the resistor vane 18 is controlled so as to increase the whirling force of the air whereby the turn down of the burner 3 can be expanded.