PURPOSE: To avoid discharge of slag floated up on molten steel surface by beforehand setting a block formed on the way of the molten steel to a bottom part range containing a discharging hole in a ladle.

CONSTITUTION: The molten steel 2 incorporated in the ladle 1 is poured into a mold 11 for continuous casting through the discharging hole 1c having structure for opening/shutting by shifting a sliding nozzle 8, a long nozzle 9 and tundish 10. The ladle 1 provides the block 4 set to the bottom part containing the discharging hole 1c in the ladle 1, forming a molten steel passage R communicated with the discharging hole 1c. Eddy of the molten steel 2 is not developed until the remained steel height in the ladle 1 comes to (h) and the remained steel quantity in the ladle 1 is as less as possible. Even in the case the remained steel height comes to lower than (h) and the slag 3 is discharged, this is detected with a detector 5 arranged in the discharging hole 1c, and it can be executed at quick correspondence that the discharging hole 1c is closed wit the sliding nozzle 8. Therefore, influence to internal quantity of a continuously cast slab due to invasion of the slag 4 is extremely little.