PURPOSE: To enhance the accuracy when the absolute angular address of a rotary encoder is read out.

CONSTITUTION: The rotary encoder is provided with an optical disc 1 having a plurality of circular concentric tracks on which a row of patterns representative of the absolute angular address is formed along with a pattern for generating a pulse signal depending on the rotated angle and judging the rotational direction. An optical head irradiates the pattern on the track with light beam, detects the reflected light beam, and scans the optical disc 1 radially with the light beam. When a rotary body begins to rotate, an optical head control section controls the light beam to reciprocate once across the optical disc in the radial direction. An absolute angular address thus read out is employed as an initial value and then the rotation angle is added to the initial value while taking account of the rotational direction thus forming the absolute angular address continuously. The row of patterns representative of the absolute angular address includes wide and narrow lines arranged annularly. The dimension of narrow line width is smaller than the diameter of light beam spot.