PURPOSE: To attain high resolution and miniaturization by recording absolute angular position information to a signal recording band at the outer circumference of a rotary disk as the permutation and combination of random optical diffraction element groups, irradiating the information recording trains as a band form through a cylindrical lens and detecting a reflected signal string from the rotary disk altogether by means of an image sensor.
CONSTITUTION: The absolute angular information pattern P a is constituted by sets of random light diffraction elements Ga in place of diffraction grating elements G in a rotary disk 11 used for the absolute type optical rotary encoder. In using the disk 11 formed with the absolute angular position information pattern P a constituted by the combination of the position of the random light diffraction elements Ga in this way, the diffracted light L d is nearly scattered light. Since the diffracted light has almost no directivity, the quantity of the diffracted light being made incident in a photodetector 14 is much decreased and made incident to all photodetectors constituting the light receiving device 14 nearly uniformly. Thus, if a diffracted light is made incident, no adverse effect is given onto a detection signal and accurate angular detection is attained.