PURPOSE: To improve the approximation precision of the center line profile of the beltlike body and to simplify arithmetic by carrying out function approximation by using a function mode which is most suitable to the actual center line profile or the beltlike body.

CONSTITUTION: When a plate material detector 32 detects the front end of a steel plate 22 to be rolled, edge sensors 22 and 24 detect the plate width Wi and edge sensors 26 and 28 begin to data on the edge position to a data processor 30 at the point of time when the front end of the steel plate 20 to be rolled is detected by a plate material detector 34. The data processor 30 calculates distances L1, L2, and L3 from a reference line A corresponding to center positions P1, P2, and P3 of the steel plate 20 from said data and further calculates the deviation distance M from the straight line B connecting measurement points P1 and P2 to a measurement point P3. Data on the distance M is transmitted to a computing element 38, which employs a correlative function mode for expressing the center line profile (y) of the plate material.