PURPOSE: To measure molten slag flowing rate even in the case of flowing in a dry pit by dipping a part of piping passing coolant to depth direction of the molten slag in slag tapping trough and arranging means for operating the slag flowing rate from temp. difference between inlet and outlet in the piping.

CONSTITUTION: The piping 2 dipping a part thereof to the depth direction of the molten slag 9 in the slag tapping trough 1 is arranged and water or coolant is flowed in the piping 2. An inlet side thermometer 3 at inlet side of the piping 2 and an outlet side thermometer 4 at outlet side are arranged. The temp. difference of the water or the coolant measured with both thermometers 3, 4 is calculated with an arithmetic unit 6 and compared with the presetted relational diagram of the temp. difference between the inlet and the outlet and the molten slag flowing rate, to calculate the molten slag flowing rate. This molten slag flowing rate is converted into an electric signal and sent to the molten slag flowing rate indicator 7. By this method, the molten metal and slag level in the hearth is accurately grasped.