PURPOSE: To detect the buckling of a steel plate running in a continuous heat treatment furnace automatically and accurately.

CONSTITUTION: Before scanned laser beam is cast from the side of steel plate 1 in parallel, the scanned laser beam is shifted for a predetermined angle with a wedge prism 9 to correct as to be introduced slantingly to the steel plate 1. As owing to the isothermal line equal refractive index line) slope on the steel plate 1, the refraction of the scanned laser beam is supressed and the beam passes in parallel, the non-sensible band of the buckling 1-1 detection decreases and thus highly accurate buckling 1-1 detection becomes possible.