To uniformly maintain the air permeability of a blast furnace by adjusting the openings of the sieve under the coke locker of the blast furnace according to the hot strength of coke.

The amt. of the powder generated by the deterioration of the coke increases when the hot strength of the coke lowers and, therefore, the air permeability of the blast furnace degrades if the opening of the sieve installed under the coke locker of the blast furnace is held intact. The opening of the sieve is, thereupon, adjusted according to the hot strength of the coke in the following manner: The opening is adjusted to enter the inside of the hatched region of Fig. according to the hot strength of the coke. As a result, the specified value is maintained without reducing the average coke grain size on the sieve and the degradation in the air permeability of the blast furnace does not arise. The grain size of the coke under the sieve is kept constant as well and there is no influence on the air permeability.