To effectively utilise reflected light by increasing the photo-detection area. [Solution] incremental photo-detection element groups (140L, 140R) are arranged in a divided manner in the circumferential direction of a rotary disk (110) on either side of a light source (130). First and second absolute photo-detection element groups (150D, 150U) are arranged on both the outside and inside in the radial direction of the rotary disk (110) with respect to the light source (130). As a result, while a continuous arrangement is maintained with regard to the first and second absolute photo-detection elements (151, 152), the incremental photo-detection element groups (140L, 140R) and absolute photo-detection element groups (150D, 150U) can be arranged so as to surround the periphery of the light source (130) from four directions.