PURPOSE: To simply and surety predict the generation or surface gloss difference generated in a metal plate.

CONSTITUTION: In a rolling mill in which the end part of a driving roll 1 is connected to a pair of right and left driving shafts 3, which are freely rotatably supported in idle bearings, with splines 2, 4, the generation of surface gloss difference generated in the metal plate is predicted based on the declination between the axial center of a male spline 2 and the axial center of a female spline 4. Concretely, by detecting distortion in the axial direction of the base of the tooth part 6 of the female spline 4, the declination between the axial center of the male spline 2 and the axial center of the female spline 4 is determined.