PURPOSE: To prevent the wear of a turbine blade by detecting directly and with high accuracy the content of conducting dust in a blast furnace gas passing through a gas turbine.

CONSTITUTION: A detecting coil 8 is concentrically installed in a bypass pipe 5c between throttle valves inserted into the bypass pipe 5c on the downstream side from a bag filter. The detecting coil 8 is integrated into a bridge circuit. The increase in self-inductance L of the detecting coil 8 by the passing of the conducting dust in the blast furnace gas is detected as that in an electric current 'i' between output terminals to1 and to2 of the bridge circuit by a galvanometer D. A value of the detected current 'id' of the galvanometer D is compared with a prescribed current value 'i0' previously set by an alarm operation circuit, and a judgement that when the detected value 'id' becomes not less than the prescribed one 'i0', the quantity of the conducting dust exceeds the prescribed one is made to operate an alarm 12.