PURPOSE: To let the detection of the charging condition of charges be surely executed by mounting solid propagating acoustic detecting ends for each unit member of movable armors.

CONSTITUTION: Movable armors 2a, 2b are disposed to the top 1a of a furnace body 1 and so operate that the charges 5 charged onto a cup 3 and large bell 4 are evenly distributed to the furnace circumference. When the charges 5 fall unevenly, the solid propagating acoustic detecting ends 10a, 10b mounted to the movable armors 2a, 2b measure the differences in sounds of the time when the charges 5 collide against the movable armors 2a, 2b. From these measurement electric signals, the presence or not of the damages of the large bell 4 and cup 3, the presence or not of any fault in the systems relating to furnace top charging, etc. are detected. These detecting ends 10a, 10b are built in with solid-gas vibration converting type microphones provided with contact protrusions which are so designed that the contact protrusions are always in linear contact with the materials to be measured. Thereby, the amounts of falling of the charges 5 in the furnace circumferential direction are quickly detected.