PURPOSE: To keep molten metal surface level in high accuracy regardless of the influence of disturbance by introducing an overshoot preventing factor and adding changing speed of a deviation in the term of I control in PID calculation.

CONSTITUTION: This method is the one for stabilizing molten metal surface level in the mold with high accuracy at the time of continuously casting a small cross-sectional billet, and at the time of controlling by the PID controller, in order to adjust the opening degree of a tundish nozzle or a stopper according to the equation, the changing speed of the deviation is added to the term of the I control in the PID calculation. In the equation, a is opening degree indicated value, a0 is opening degree reference value, H0 is molten metal surface level setting value, is mold molten metal surface level measured value, K10 is overshoot preventing factor, KP is proportional control factor, KI is integral control factor, KD is differential control factor. By this method, the molten metal surface level is controlled with high accuracy and this control system can speedily be responded to the rapid fluctuation.