PURPOSE: To make preheating work efficient by adjusting an electric damper opening degree based on measured results of pressures in a molten steel vessel and preheating box and controlling so that difference between the pressures in the molten steel vessel and the preheating box always becomes the constant.

CONSTITUTION: A burner 2 for preheating the vessel is arranged at the molten steel vessel 1 having a submerged tube 3, and the submerged tube 3 is covered at the preheating box 4 to preheat the submerged nozzle 3 with the burner 5 for preheating the nozzle. Then, the pressures in the molten steel vessel 1 and the preheating box 4 are measured with pressure sensors 16, 17, and the electric damper 8, etc., is controlled with the detected signal and the pressure in the molten steel vessel 1 is adjusted and the difference with the pressure in the preheating box 4 is made to always the constant. By this method, as high temp. frame for preheating can be stably supplied in the submerged tube 3, the work can be surely and efficiently executed.