PURPOSE: To detect a furnace charge such as ore and coke in the high-temperature lower area of the furnace stable or a long period, by radiating a radio wave to the furnace charge, receiving its transmitted or reflected electric power, and detecting a charge layer from the difference of the received electric power.

CONSTITUTION: Waveguides 51 and 52 coupled with a microwave transmitter 8 and a receiver 9 are provided at the detector part 2 of the blast furnace, and openings of the waveguides 51 and 52 are in a horn type antenna shape and provided in a furnace wall 7 while facing each other horizontally. Those waveguides are cooled with the cooling water passed through cooling housing 61 and 62 and further cooled with gaseous nitrogen blown in the waveguides 51 and 52 while purged so that neither coke nor ore enters the opening. When the openings are set 20cm away from each other, the received electric power is almost zero (-86 dB) in the ore layer and -40W-50 dB in the coke; the difference between the both is 40W50 dB, which is clear.