PURPOSE: To promote floatability of inclusion while preventing enclosure of powder even under any casting condition and to stably obtain high clean steel having only a little inclusion at good yield by independently setting electromagnetic force to be given to molten steal flow from a submerged nozzle to right and left sides in accordance with casting condition.

CONSTITUTION: U-shape electromagnets constituting with coil 8 and yoke 7 at a outside of a mold 4 are set separately to the right and left sides adjoining the submerged nozzle 1 and each electromagnet is independently set. Therefore, as showing in the figure, in the coil at left side having rapid molten steel flow speed from a discharging hole 2, the electromagnetic force 10 is impressed so sufficient as to damp the flow speed. On the other hand, in the coil at right side having slow molten steel flow speed, by impressing a little weak electromagnetic force, consequently, both flows at right and left sides can be maintained to the ideal condition. Judgement, whether the impression of the electromagnetic force is suitable or not, can be decided by measuring the molten steel surface levels near short side mold and near the submerged nozzle each at right and left sides at the same time with four molten steel surface level sensors 6 set on the molten steel surface in the mold 4.