PURPOSE: To detect the behavior of the raw material charged into a blast furnace and to enable stable operation of the blast furnace by directing and disposing the receiving part of a radiometer to and in the blast furnace and measuring the radiation energy of the microwave band emitted by the raw material.

CONSTITUTION: Mounting holes 3, 4, 5 are formed circumferentially at a specified interval to the circumferential wall 2 of a blast furnace 1 in the height direction. Probes 6a, 7a are inserted into the holes 3, 4 in such a way that the top ends thereof do not project into the furnace 1 and antennas 6b, 7b are coupled thereto to apply received μ waves to μ wave radiometers 6, 7. A probe 8a is freely attachable and detachable and loosely inserted into the hole 5 so as to advance into the furnace 1 and the antenna 8a is attached thereto to apply the received μ wave to a μ wave radiometer 8. A calculator 9 detects temp., layer thickness ratio, charge descending speed, temp. distribution and grain size distribution from the signals transmitted thereto from the radiometers 6, 7, 8.