To provide a detecting method for a chain extension rate which can accurately detect a chain extension rate in spite of the variation of a load given to a chain and its polygonal motion and detecting device used for its practice.

A laser light radiated onto the peripheries of rollers 85, 85 is scattered by slight roughness of their circumferences, and the scattered light enters picking up devices 1, 2, and an image having an circular arc-shaped image due to the scattered light is formed on CCD of the picking up device 1, 2. A computer 5 computes the center positions C1, C2 of images given from the picking up devices 1, 2, and computes distance 11, 12 from the center positions C1, C2 of the images to their end on another image side to obtain an actual distance M between center positions. The computer 5 obtains a difference between the actual distance M between center positions and a reference distance between center positions, and compares the rate of the obtained difference to the reference distance between center positions with the threshold value, and in the case where the former is larger than the latter, an output device 6 outputs an alarm for a chain exchange.