To reduce errors in angle detection caused by the deformation or the decentering of the axis of rotation, etc., of a rotator in a simple constitution.

During the rotation of a rotator 12, contrast patterns on the tape 14 on the circumferential surface of the rotator 12 is irradiated with light from a light source part 16. On the basis of the reflected light, the length of an arc corresponding to the angle of rotation of the rotator 12 is detected at angle detecting parts (18 and 20). At this time, a displacement (h) from a reference location on the circumferential surface of the rotator 12 is detected at a displacement detecting part 22, and a displacement signal corresponding to the displacement is outputted. Therefore, in the case where the self-weigh deformation, the decentering of the axis of rotation, etc., of the rotator 12 occur, the length of an arc including errors caused by them is detected by the displacement detecting parts, the above mentioned (h) is detected by the displacement detecting part 22, and a displacement signal corresponding to the displacement is outputted. Then on the basis of the displacement signal and the length of an arc detected above (including errors), the angle of rotation of the rotator 12 in which the above-mentioned errors are canceled is calculated.