Using one rebelgye, primary, secondary, and bee jingseong bath surface (湯面) extracts only the variations, and high accuracy by using the bath surface of the bath surface level control that it should be essentially controlled by extraction, remove the standing-wave components of the tertiary in order to achieve the level control, shows a model of the bath surface level variation caused by the natural standing wave oscillating cycle of a response to the mold width (12, 14) to the secondary vibration system, a level measurement value measured by the bath surface rebelgye and models (12, 14) models the variation and at least one of its derivative of the output of 12, 14 models (12, 14 by feedback to the input of) the excitation (勵 振), and the output of the resulting model (12, 14) with by also estimating the bath surface level variation component caused by the standing wave, it has a deviation of the output of a level measurement value measured by the bath surface rebelgye and models (12, 14) and a level signal to remove a standing wave component, using the art-level signal by the feedback control it has to operate the actuator for controlling the flow of molten steel entering the mold.