PURPOSE: To prevent intrusion of slag into the molten metal to be tapped from a converter by detecting the light emitted from the molten tapped from the converter and actuating surely a slag cut from the temp. difference between the temp. stable region and slag intrusion region of the moten steel.

CONSTITUTION: A titled method for detection consists of a stage 11 for detecting the light emitted from a molten steel 4 during tapping from a converter 1 and converting said light to an electrical signal, a stage 12 for amplifying said signal to the 1st and 2nd electrical signals, a stage for sampling the above-mentioned 1st electrical signal, a stage for generating the holding signal of the level when the sampling value maintains the same value and a stage for stopping the tapping. The above-mentioned stage for stopping the tapping actuates when the detected difference in level between the above-described holding signal and the 2nd signal exceeds the prescribed value. The molten steel 4 in which slag 5 is not intruded is thus obtd.