PURPOSE: To prevent a break-out and an overflow from a casting mold by providing a circuit for cutting off a DC signal component, between an oscillator of an injecting device and a driving device.

CONSTITUTION: A molten metal surface position measuring instrument 5 is provided on a casting mold 4, and also a measuring signal (a) is inputted to a regulator 9. A molten metal surface position setting device 8 inputs a reference molten metal surface position signal (b) to the regulator 9 in the same way. Between an oscillator 10 and a driving device of a sliding nozzle 1, two R-C circuits consisting of a capacitor and a variable resistor are provided. When a molten steel 2 in a tandish 3 is being injected to the casting mold 4, even if a fault is generated in the oscillator 10, namely, even if an oscillating signal (d) of DC of a high level is transmitted continuously, a phase regulator 30 pro vided with a capacitor is in the output side of the oscillator 10, and its output becomes 0V immediately. Accordingly, an opening adjustment of the nozzle 1 is returned to a controlled state before an excitation, and a break-out, etc. are prevented.