PURPOSE: To detect surely slag-off without receiving the influence owing to the opening and closing of a sliding nozzle, etc. by providing a standard electrode and a molten steel-slag electrode in a nozzle from a ladle to a tundish, and detecting the difference in the amts. of the oxygen in the molten steel and the slag.

CONSTITUTION: Molten steel or slag MS passes both long nozzles 1, 2 under a ladle from the ladle and flows down into a tundish. An standard oxygen electrode 8 and the MS are conducted through the zirconia 6 on the MS side in the space of a circular truncated cone shape enclosed with the insulation refractories 5 lining the lower wall part of a trunnion 3. On the other hand, a molten steel- slag electrode 10 which is grounded to a part of the trunnion 3 and a is penetrated through the nozzle 2 in the case in which the nozzle 2 is conductive and non-conductive is provided. Said electrode is conducted with the MS through the trunnion 3 and the nozzle 2 thereby forming an oxygen concn. cell using the MS as an electrolytic material. The amt. of the dissolved oxygen in the molten steel differs from the amt. in the slag and therefore if the MS changes from the molten steel to the slag, electromotive force is generated between the electrodes 8 and 10, and the slag off is surely detected from the same.