PURPOSE: To eliminate the dispersion of a sealing work and to save the labor for work necessary for the sealing and casting preparation.

CONSTITUTION: A sealing device 1 is arranged together with a hot blast blowing hole, and a sealing material supplying nozzle 10 for discharging the sealing material 7 into a mold 2 and a sealing material supplying part 8 for supplying the sealing material into this sealing material supplying nozzle 10 are provided. The sealing material supplying nozzle 10 can be shifted along the peripheral edge of the mold 2 and can be ascended/descended along a mold copper plate 3. Then, the molten steel poured into the mold 2 at the time of starting the casting in a continuous casting is easily and surely prevented from leaking and flowing out from gap between a dummy bar head 4 and the mold copper plate 3, because the sealing material hardened at high temp. by the hot blast without man-power automatically plugs the gap.