PURPOSE: To allow surely a molten metal to remain in a vessel at a planned amt. with less waste by detecting the thickness of a slag layer by a heat resistant float which utilizes the difference in specific gravity between the molten metal and slag and calculating the amt. of the molten metal in accordance with such information and the sectional area of the vessel.

CONSTITUTION: The 1st processing unit 13 calculates the thickness A of a slag layer and outputs the same to the 2nd processing unit 14 when the deflection point is detected. The unit 14 reads the height information of the heat resistant float 7 at the deflection point P1 determined from a load cell 10 from a rotary encoder 12. Said unit extracts the sectional area of a ladle 1 in which the slag layer I exists from the preliminary written information on the sectional area of the ladle 1 according to such positional information. The total weight of the slag is then calculated and the total weight of the molten metal is calculated from the net weight obtd. by removing the tare from the output of the cell 10.