PURPOSE: To obtain the true conductivity between electrodes and accurately measure molten steel level by obtaining the total resistance value in the closed loop formed by shorting between the electrodes immediate before the probe enters slag and subtracting the same from the actually measured resistance value.

CONSTITUTION: When an elevating winch 10 is operated to raise an elevating truck 8 to the specified height, a reset switch 23 actuates, causing a pulse counter 21 to start counting. When the truck 8 reaches the 2nd position, the winch 10 stops operating and an electrode shorting device 24 starts operating to short a pair of the electrodes of a probe 11 with its shorting plate, thereby letting a closed loop be formed. Slight current is flowed in the closed loop and the total resistance value in the closed loop is measured with a zero point adjusting device 19. After removal of the shorting plate, the winch 10 is operated again to lower the probe 11, and the subtraction of the closed loop resistance value from the measured resistance value of the device 19 is accomplished by a boundary point discriminator 18. When the differences between the resultant resistance value between the electrodes and the reference resistance value become zero, it is decided to be molten steel level.