An optical system for detecting creases has been developed. The system consists of a movable sensor head with a pulsed semiconductor laser of 10 kHz and a silicon photodiodes , an analog data processing system including an amplifier with a band-pass-filter, a rectifier and an integrated circuit , and a digital data processing system including an 8 bit A/D converter with a sampling frequency of 1 kHz , a personal computer, and an alarm for crease detection. The detection capability for creases with widths of 2 mm and lengths longer than 3 m was above 95 % for manufacturing plants with production conditions of lm/s and sheets with widths of 1 m.