PURPOSE: To enable accurate molten steel surface level control reduced in error by learning a control gain in consideration of the looseness of a driving system of a flow rate adjusting member and the depth of molten steel in a tundish and adjusting the opening degree of a stopper.

CONSTITUTION: The surface level of molten steel 2 in a mold 4 is measured by a molten steel surface level meter 12, and this measured value is outputted to a level controller 10 and a gain learning computing element 11. At the time of occurring the difference between this measured value and the aimed molten steel surface level value inputted from the outer part, an opening degree command variation in one time of the control period to a cylinder 9 so that the difference becomes zero, is calculated by the level controller 10. Further, a corrected control gain is obtd. by using a learnt flow rate gain inputted from the gain learning computing element 11 and a flow rate gain in a design value stored in the level controller 10. An actual opening degree command variation in one time of the control period outputted to the cylinder 9 is obtd. by multiplying the opening degree command variation by the corrected control gain to drive the cylinder 9, and the molten steel surface in the mold 4 is held to the aimed level by adjusting the opening degree of the stopper 7.