Studies on fluctuation of mold bath level were carried out to obtain better surface quality of cast slabs.(1) The fluctuation occurs due to delay in the movement of slide gate caused by adhesion of solidified steel.(2) By the vibration of slide gate, its movement is improved and the fluctuation of bath level is remarkably decreased.From these results, the new technology for the automatic bath level control system was developed. A special feature of the system is the addition of a vibratory signal to the control signal of the slide gate.The system has been applied to all casters in Wakayama and Kashima Steel Works and as a result, the yield of cast slabs have been remarkably improved