PURPOSE: To detect the outflow of slag with the accuracy of the same level as in the prior art and to assure various operations without trouble during continuous casting by using a reflection mirror for receiving the thermal radiation energy of the molten metal flowing down in an immersion nozzle.

CONSTITUTION: A reflection mirror 9 having an elliptical shape is horizontally installed on the outside of an immersion nozzle 5 attached to a sliding nozzle 3 installed in the bottom of a ladle 1 in such a way that the one focus thereof is positioned at the axial center of the nozzle where the molten metal flows. A receiving antenna 10 of which the detecting direction is directed to the center of the mirror 9 is installed at the other focus of the mirror 9. The thermal radiation energy of the molten steel (m) and slag S flowing down in the nozzle 5 are received by such mirror 9, the antenna 10 and a radiometer 11 connected to the antenna 10. The outflow of the slag S is detected from the change in the thermal radiation energy thereof.