PURPOSE: To control the diameter of bubbles in molten metal by impressing static magnetic field orthogonal to the developing direction of the bubbles and AC current in the developing direction of the bubbles.

CONSTITUTION: The molten steel 2 is filled in a ladle 1, and by blowing gas G from a nozzle 3, the bubbles 9 are developed. At the side wall surface in a bottom part of the ladle 1, one pair of static magnetic field generating devices 4, 5 are arranged to generate the static magnetic field from the N pole toward the S pole in the horizontal direction. By AC power source 8, the AC current is conducted in the molten steel 2 in the bubble developing direction through an electrode 6 dipped into the molten steel 2 above the nozzle 3 in the ladle 1 and an electrode 7 around the nozzle 3. By changing the frequency of the AC current, the diameter of the bubbles can be controlled.