a device for measuring a rotation angle of a rotary element has first , second and third photo - electric sensors , and a rotary disk having first and second circular arrays of openings , each radial edge of the first array of openings being adapted to be detectable by the first photo - electric sensor so as to generate a first electric signal alternating between “ on ” and “ off ” each time when the rotary disk rotates each five times of a unit angle , each radial edge of the second array of openings being adapted to be detectable by the second and third photo - electric sensors so as to generate second and third electric signals , respectively , the second and third signals each alternating between either “ on ” and “ off ” or “ off ” and “ on ” each time when the rotary disk rotates alternately two times and three times of the unit angle , the “ on ” and “ off ” alternations of the first , second and third signals being shifted from one another by a multiple of the unit angle , wherein the “ on ” and “ off ” alternations of the first , second and third signals successively occurs are counted to obtain a rotation angle of the disk , and various irregularities in the occurrence of the “ on ” and “ off ” signals are checked by the order of occurrence thereof in each set of five consecutive occurrences thereof .