To highly accurately calculate a bulging frequency in real time, and to highly accurately control bath surface levels of molten steel in a mold on the basis of the calculation result.

A disturbance compensator 83 extracts variation components of the bath surface levels at a plurality of specified frequencies f_i by returning a difference between the bath surface levels which are measured by a bath surface level indicator and outputs of frequency models 831a to 831e to an output of each frequency model, the specific frequency f_i corresponding to the variation component of the bath surface level whose amplitude is the largest out of the extracted variation components of the bath surface levels is determined to be the bulging frequency, and a disturbance compensation value is added to an opening command value so as to compensate the disturbance of the specified frequency f_i. By this, since a frequency analysis in a band region in the vicinity of the bulging frequency can be performed without using a frequency analysis operation technique such as FFT, the bulging frequency can highly accurately be calculated in real time, and the bath level of the molten steel in the mold can highly accurately be controlled on the basis of the calculation result.