PURPOSE: To intensify rotary-stirring of molten steel in a tundish and to improve separating effect to inclusion in the molten steel by constituting polarities of electrodes so as to mutually differ at the facing position across a rotary vessel.

CONSTITUTION: By using the tundish 10, the molten steel is continuously poured to produce a cast slab. To the tundish 10 having 1m of the inner diameter of rotary vessel 10a, coil devices 1c, 1d for generating shifting magnetic field, having 1m the inner diameter and 1.6m the outer diameter to the coil devices 1c, 1d in the rotary center 29 of molten steel in the rotary vessel 10a, is set, and current having 2000A with 3Hz, is impressed. Each electrode for forming the coil devices 1c, 1d is set as facing to mutual electrodes at plactically symmetrical position to the rotating center 29 of molten steel in the rotary vessel 10a so that the polarities of these facing electrodes mutually differ from each other. By this method, the rotary-stirring of molten steel in the tundish 10 is intensified and the separating effect to the inclusion is made to high and the cast slab having good quality is obtd.