PURPOSE: To enable prediction of breakout by disposing plural ultrasonic probes to a casting mold, detecting the wave reflected from the surface of a billet, monitoring the fluctuating period of the detection level and comparing the relative speeds of the casting mold and the billet with a drawing speed.

CONSTITUTION: A molten metal 2 is supplied from a tundish 1 via an immersion nozzle 3 to a casting mold 7 oscillated vertically at a specified period by a driving device 16. A billet 4 is drawn via powder 12. Plural ultrasonic probes 8a, 8b are disposed in the vertically different positions of the mold 7 to monitor the characteristic on the surface of the billet. Ultrasonic waves are transmitted therefrom toward the billet 4 and the waves reflected from the respective parts of the billet surface are detected. The mutual correlations between the time series signals of the detection levels of the respective reflected waves are thus obtd. The difference in the detection time between the probes 8a and 8b is determined from said correlations and the relative speeds of the mold 7 and the billet 4 are calculated. Then the calculated value and the drawing speed are compared. The breakout is thus predicted with high accuracy.