Reflected light can be effectively utilized by increasing a light receiving area. Incremental light receiving element groups are separated and arranged in the circumferential direction of a rotating unit while placing a light source therebetween. First and second absolute light receiving element groups are arranged at both sides of the outside and inside of the light source in the radial direction of the rotating unit. As a result, first and second absolute light receiving elements are continuously arranged, and also the incremental light receiving element groups and the absolute light receiving element groups can be arranged to surround the periphery of the light source from four directions.