PURPOSE: To cast a steel ingot always at the specified thickness and to obtain the good-quality steel ingot which is short in a non-stationary part and has no inclusions by minimizing the initial space of a horizontally moving mold and controlling the moving speed of the casting in such a manner that the molten steel level is kept constant at the same instant when pouring is started.

CONSTITUTION: A molten metal level detector 11 is installed above the horizontal moving mold 2 to control a driving motor 6 via a controller 12. A stopper 7 is held open and the motor 6 is controlled simultaneously with the starting of pouring to control the movement of the mold 2 and the casting is thus executed without regulating the flow rate of a tundish nozzle 8. The mold speed is controlled to meet the flow rate of the nozzle changing by various reasons so that the molten steel level is maintained always constant in the stationary state. The moving speed is made small in the initial period of casting and is controlled to the gradully increasing speed. The non-stationary part is thereby made extremely short.