PURPOSE: To measure the level of a molten steel with high accuracy with an inexpensive sensor having a simple construction by detecting the erosion condi tion of a cable sensor.

CONSTITUTION: The sensor 10 having a length l0 is inserted into a vessel 50 by a feeder 42 and in this stage, a proximity sensor 36 generates output at the point of the time when the front end of the sensor 10 passes in front thereof. The position of the sensor 36 is known and is designated as a reference level L0. The part entering the molten steel 52 is eroded when the sensor 10 enters the molten steel. The erosion is known from the decrease of the sensor length. The insertion length Δl0 of the sensor from the level L0 is known by counting 26 the output pulse of a pulse generator 40 from the point of the time when the output is generated in a detection part 34. The erosion length Δl1 is deter mined as l0-l1 from the present lengh l1 of the sensor 10 and the level L1 of the molten steel can be determined by making a calculation 28 of Δl0-Δl1. A rangefinder 44 irradiates laser light on a slag layer 54 and detects the surface position of said layer. The thickness of the layer 54 is determined by calculating 30 the surface position-of the layer 54-the surface L1 of the molten steel.