A method measures the flatness of a metal product and an associated device. The method applies to a metal product, in the form of either a strip or a plate from a metallurgical processing line. The product to be measured being, by default, free of external traction. The method contains the following steps: a) illuminating a portion of a face of the product under uniform intensity; b) capturing an image of a light line of the illuminated portion; c) relatively moving the illuminated portion and the light line in a defined direction in relation to the product; d) repeating steps a), b), c); and e) collecting the images of lines in a two-dimensional distribution of intensities and selecting a strand direction of the product in which, if at least one wave of intensities is detected, a local amplitude variation of the wave delivers a local strand flatness defect value.