To provide a device for protecting a polishing plate holder of a polishing plate having a dovetail groove even when overload is applied to the polishing plate for restraining a material to be straightened during straightening work in a roll straightening machine for steel.

The polishing plate is fixed by fixing a cotter 4 for preventing fall-off of the polishing plate to the polishing plate holder 3 for inserting into and fixing to the polishing plate 2 of the roller straightening machine with a cotter pin 5 and by inserting and fixing the tenon 2a of the polishing plate into and to the polishing plate holder having a dovetail groove 3a. The shaft part of the cotter is provided with a cotter pin locking hole 4e for preventing the fall-off of the cotter, and a projecting surface for locking by bringing into contact with the polishing plate in a direction perpendicular to the shaft part of the cotter, and it is brought into contact with the side end part of the polishing plate. A notched part is provided on the root of the projecting surface for locking the polishing plate and, when the rotational propulsive force of the material to be straightened increases, the cotter is broken and the polishing plate holder is protected.