PURPOSE: To always control furnace condition stably by arranging sensors for detecting the sound and vibration onto a furnace wall and deciding distributing state of raw material in the furnace with these detected values at the time of producing molten iron by melting the raw materials of iron ore, scrap, coke, etc., in the cylindrical melting furnace.

CONSTITUTION: The raw materials of the iron ore, coke, scrape, slag-making material, etc., are charged to a cylindrical melting furnace 1, and hot blast, fine powdered coal, etc., are blown from tuyeres and the raw materials are melted and reduced to produce the molten iron. In this furnace, measuring instrument 2 providing bar 2b, wire 2a for measuring height H of the charged raw material, is arranged and also the sensor parts 3 providing the sound sensors S1 and the vibration sensors S2 at outside of the furnace wall under shifting the height positions thereof, are set in two steps at the upper and lower positions. Outputs of these sensors 3 are taken out as the prescribed frequencies of sound signal and vibration signal through filters G1, G2 and read in FFT processors 12, 13, respectively. These are inputted into an operating processor 15 and the distributing condition for each kind of the charged material in the furnace is obtd. and by the measured value of the height H of raw material surface, the charge of raw materials and the blowing of hot blast, fine powdery coal, etc., from the tuyere hereafter, are controlled to operate the furnace under stable condition.