A rotation detector includes a first distance sensor disposed to face a rotating body with a gear portion formed non-parallel to a rotation axis for continuously measuring a distance from the gear portion during rotation of the rotating body, a second distance sensor disposed at a position different from the first distance sensor in a direction of the rotation axis for continuously measuring a distance from the gear portion during rotation of the rotating body, and an axis shift detecting portion for detecting an axis shift of the rotation axis based on a variation in the distance measured by at least one of the first and second distance sensors.