To effectively prevent an out-of-step by an abrupt load increase by correcting the opening/closing command of the opening/closing means installed to a pouring nozzle to a continuous casting mold by an out-of-step avoidance circuit when the out-of-step is detected by an out-of-step decision circuit in accordance with the deviation between the rod position and target position of a stepping cylinder.

A sliding nozzle 2 for adjusting the pouring rate by the sliding is disposed in mid-way of the pouring nozzle 3. The gap thereof is adjusted by the advance and retreat of the output rod 6-1 of the stepping cylinder 6. A molten metal level is detected by a level detector 5 and is controlled by controlling the sliding nozzle 2. The deviation between the present rod 6-1 position of the stepping cylinder 6 and the target position thereof is detected by a motion detector 9. In the case that the out-of-step by the load increase is detected by the out-of-step decision circuit 10, the opening/closing command is outputted to the out-of-step avoidance circuit 11 and is corrected by the same. The instance that the out-of-step does not cease in spite of gap inching is decided to be trouble. The cooling and solidifying of the molten steel in the casting mold 4 are stabilized, by which the quality of the slab is improved.