PURPOSE: To obtain qualitative information, according to which a planar shape is accurately detected even in the case of a hot reversible rolling machine so that not only the propriety judgement of the fatness of the plate member but also the control of the rolling rolls can be accomplished, by making such an optical beam to be projected upon the plate member that is extends in the longitudinal direction of the same.

CONSTITUTION: Two optical beam projectors 1a and 1b are arranged above the conveyor of a plate member M being rolled such that the projector 1a projects an optical beam upon the widthwise center of the plate member whereas the projector 1b projects an optical beam extending in the vicinity of one side edge in the conveying direction. The optical image appearing on the surface of the plate member M is picked up by an image pick-up device 3 such as an ITV. A plate width setter 4 accomplished the position control of the projector 1b in the widthwise direction through a controller 5 on the basis of the preset information. The controller 5 controlls the respective operations in response to the detecting signals of the detector 6. The output of the device 3 is turned digital by an optical image processor 7 and is displayed in a monitor image receiver 9. On the other hand, a flatness recognizing device 10 compares the digital signals with a preset standar value to judge the propriety of the flatness.