PURPOSE: To contrive saving of consumption fuel rapidly by keeping a discharge reference value of NOx strictly, by varying a quantity of supply pure oxygen by comparing a density/value of NOx in burnt exhaust gas with a discharge reference value of the NOx by detecting the NOx density value always.

CONSTITUTION: Density of NOx in exhaust gas detected by an NOx meter 17 mounted on a stack 16 of a heating furnace 2 of a piece of steel is applied to a selector 19 through a converter 18 after an electric current has converted. On the one hand, a value obtained by making current conversion of a value obtained through a convertor 21 by making an analysis of oxygen density in combustion air after it has been enriched by making use of an oxygen analyzer 20 is made to keep applying to the selector 19 also. A value of NOx applied to a NOx adjusting meter 23 through the selector 19 makes a quantity of supply oxygen reduce by controlling a flow adjusting valve 24 so that closing operation of the flow adjusting valve 24 equipped on an oxygen line 9 is made when a measured NOx value detected by an NOx meter 17 is higher than a discharge reference value. In addition to the above, when the case is just opposite to the above, quantity of supply oxygen is increased.