PURPOSE: To coincide the center of Side-Trimmark with the cente of steel plate to be trimmed and to move a pay-off reel for the lag space between the center of steel plate and the side-trimming-line center so that the deviation correction can be done accurately.

CONSTITUTION: A steel plate location detector 11 is placed at the inlet side of Side-Trimmark. The detector 11 detects the lag space X between steel plate center A and side-trimming-line center B, and the detecting signal is compared and operated by compare-operation device. The operated value is input into a control device; as the pay-off reel 2 is moved in the direction to absorb the lag space X, a cylinder 15 for moving the side-trimmer is operated; thereby the center of Side-Trimmark is conincided with the center A of steel plate by moving Side-Trimmark for the lag space X.