PURPOSE: To control a molten steel surface level in good control accuracy regardless of clogging condition in a sliding nozzle.

CONSTITUTION: In a nozzle clogging index calculating part 91, the clogging condition in the sliding nozzle is estimated based on the difference of the molten steel surface level obtd. relating to the molten steel level detected with a molten steel detector, the opening degree of the sliding nozzle detected with an opening degree detector and casting speed detected with a casting speed detector. In a control gain selecting part 92, the optimum control gain in the optimum control rule preset so as to obtain the optimum opening degree adjusting rate in the sliding nozzle is formed based on the estimated clogging condition in the sliding nozzle. Further, in a nozzle opening degree command changing rate calculating part 93, the above-mentioned optimum control gain is set to the optimum control rule, and by using this optimum control rule, the optimum opening degree adjusting rate in the sliding nozzle is obtd. based on the detected results of the molten steel level, the opening degree of sliding nozzle and the casting speed.