PURPOSE: To provide an optical method detecting rolled plate shape by which the virtual image of a rod-like light source projected to a rolled plate is processed precisely at a high speed in the stage to process the image and the shape of a rolled plate can be detected well.

CONSTITUTION: In rolling process of a plate, light rays are radiated to the plate from a rod-like light source 2 installed along the width direction and the virtual image of the rod-like light source projected to the plate is photographed by a camera. The image is divided into a plurality of areas and data of each divided area is converted into binary data and based on a function, the position of the obtained bright lines in the longitudinal direction of the plate is calculated for each divided area.