PURPOSE: To detect a slag flowout with high accuracy by simple device by detecting the vibration of the case of a molten metal and slag being subjected to a mixed flow inside the long nozzle of a ladle with the difference in the gas pressures caused between the gas discharging pipe lines oppositely arranged equally on the outer periphery of the long nozzle.

CONSTITUTION: The gas discharging pipe lines 2, 2 in plural couples of an equal structure which blow an air or gaseous body onto the surface of the long nozzle 1 fitted vertically on the ladle are symmetrically arranged. The outlet side orifice 4 is provided at the outlet port of the gas discharging pipe lines 2, 2 and the orifice 5 inside the pipe line is provided at the inner side of the pipe line. A gas pressure detector 6 is provided in the space between the outlet side orifice 4 and pipe line inside orifice 5. The vibration of the long nozzle 1 is caught by the difference in the gas pressures detected by the gas pressure detector 6 of the pair of gas discharging pipes 2 relatively arranged in the long nozzle peripheral direction, and the slag flowout is detected.