To retain an accurate operation by enabling an operator with a special qualification to maintain a measuring instrument.

In a method for operating a furnace 1 with a smoke pipe 11 for unloading smoke, a means 19 for introducing surrounding air into the smoke pipe, and a smoke extractor 16 that is arranged in the smoke pipe at the downstream of the means 19, the temperature of the smoke is measured at a first point 31 near an exit 9 of the furnace and a second point 33 in the smoke pipe at the downstream of the first point 31, temperature that is measured at the second point is subtracted from the temperature being measured at the first point, the subtraction result is compared with a positive or zero data value ΔT, and the ratio of the flow rate of a fuel for the flow rate of an oxidizer being introduced to the furnace 1 is subtracted when the subtraction result is smaller than the data value ΔT.