PURPOSE: To detect the configuration of the section of a steel plate with a high accuracy by constituting a detector by a photosensitive element, a rotary reflecting mirror, position detecting means, arithmetic means and the like.

CONSTITUTION: When a steel plate 10 is conveyed to a roller table 14, central portion detecting unit 18 and a side detecting unit 20 arranged approximately in a line along the transverse direction of the steel plate are operated at the same time. The unit 18 measures a distance L1 between the unit 18 and the approximately central portion of steel plate 10 and the unit 20 measures a distance P between the reference position of photosensitive element 42 and a laser beam receiving point and the rotary angle of a rotary mirror 32, measured data being fed to arithmetic unit 48. The unit 48 conducts an operation to calculate coordinates L and R from a distance R in the direction of the width of the steel plate and a distance L from the focal point of a lens. Thus, the unit 20 detects the configurations of the surface portions of the margins of the steel plate 10 in its width direction and measures the range of the predetermined regions.