PURPOSE: To prevent not uniform solidification and surface crack by controlling a taper quantity of the inner peripheral face of lower part of mold corresponding to the contacting condition between the solidified shell of molten metal and inner peripheral face of mold obtained from measured result in operation in continuous casting of cast billet.

CONSTITUTION: In continuous casting of a cast billet, the upper part in the casting direction of mold is constituted with a fixed tapered part 1 and the lower part with a variable tapered part by many vertically directed mold segments, after a solidified thickness at the fixed tapered part 1 is made to ≥5mm, the heat flow flux between solidified shell and mold at the variable tapered part 2 is detected. Based on the detected signal, a load quantity to load applying devices of a hydraulic cylinder and stepping cylinder 5 arranged facing each mold segment is controlled, the forced deformation to each mold segment is given so that the tapered quantity is become optimum corresponding to solidification shrinkage to be changed by a kind of steels and casting conditions, the solidified shell and inner peripheral face of mold is kept in appropriate contact.