A rotation angle detector includes a rotator, first and second detecting bodies, an auxiliary detecting body, first and second detectors, an auxiliary detector, and a controller. The first detecting body rotates in association with the rotator, and the first detector detects rotation of the first detecting body. The second detecting body rotates in association with the first detecting body at a different speed therefrom, and the second detector detects rotation of the second detecting body. The auxiliary detecting body is engaged with the rotator at a position symmetrical to the first detecting body with respect to a rotation axis of the rotator. The auxiliary detector detects rotation of the auxiliary detecting body. The controller detects a rotation angle of the rotator based on detection signals from the first, second, and auxiliary detectors.