PURPOSE: To enable a continuous ingot free from structural defects to be cast by constituting part of a mold with a movable piece and controlling the rate of taper of mold surface through continuous casting process so as to eliminate the gap between the continuously cast ingot and said surface.

CONSTITUTION: The upper part of the short side 1 of a mold is connected to a short side moving base 4 rotatably by means of a pin 11, and the lower part is connected to a piston rod 12 of a hydraulic (or air) cylinder 6. A slight gap G is provided between the short side 1 and the mold long side 3 so that the taper of the short side 1 may be smoothly varied by the cylinder 6. The thrust of the cylinder 6 is set constant in such a manner that is equals to the pressure by the static iron pressure of the time when an ingot 10 prefectly contacts the short side 1. Hence, if any gap exists between the short sides 1, 2 and the ingot 10, the force that the short side 1 receives diminishes and the short side 1 moves until it contacts the ingot 10. If it becomes larger than the set value, the short side 1 retreats to the optimum position. However, this is so accomplished that the rate of the movement of the short side 1 maintains a permissible taper region.