PURPOSE: To detect behaviour of raw material fed to a blast furnace exactly for a long period by protruding a top end of a receiving part of a microwave radiometer into a blast furnace and providing an operation means by catcying microwave energy radiated from the charged raw material.

CONSTITUTION: Receiving parts 41, 42 are made to penetrate through attaching holes 31, 32 formed on a furnace wall 2, and their top ends are located to by ≥ca.50W100mm to the furnace core side than the mixing layer 10c. The opening at the top end of the receiving parts are closed by lids 7 made of a refractory which permits penetration of μ wave. The radiant energy of the μ wave emitted from the charged raw material 10 through waveguides 41a and 42a of the receiving parts 41, 42 are detected by antennas 41C, 42C and detection signals are given to the μ wave raiometer 51, 52. The behaviour of the charged raw material is detected by an operator 6 by measuring the μ wave energy in accordance with the lapse of time. For example, ratio of time measuring high energy to time measuring low energy is operated, and thus, ratio of thickness of sintered ore 10b to the thickness of coke layer 10a is detected.