PURPOSE: To accurately and quickly hold the molten metal surface level in a mold to an aimed molten metal surface level by accurately obtaining a mechanical backlash existing between a slide valve and a piston rod in a hydraulic cylinder for adjusting the opening degree of valve based on the variation of the molten metal surface level in the mold by working the hydraulic cylinder at the initial stage of casting without developing bulging phenomenon disturbing the variation of the molten metal surface level, and working the hydraulic cylinder while automatically correcting this mechanical backlash.

CONSTITUTION: At the initial stage of casting, the piston rod effective shifting distance contributing to the variation of the molten metal surface level is obtd. from the variating quantity of the variating velocity of the molten metal surface level at the time of working the piston rod 11a in the hydraulic cylinder 11 by the fixed setting shifting distance and thereafter, working so as to return back to the original position after passing the fixed setting time, a cross sectional area of the mold 56 and a flow rate factor of the molten metal flowed in an immersion nozzle 57. A backlash correcting value arithmetic part 31 is provided for obtaining the mechanical backlash from the actual piston rod shifting distance based on this effective shifting distance and the setting shifting distance.