A rotation detection device includes a light emitting portion, a light-passing control member, and a light receiving portion. The light-passing control member controls passing of the light from the light emitting portion. The light receiving portion outputs a light receiving signal based on the light received by the respective light receiving elements as detection signals representing detection results of the rotation by the rotator. A first virtual extended line that extends the first opening margin and a light-receiving-portion center line along the first direction passing through the respective light receiving elements at the light receiving portion pass through positions shifted from a rotational center of the light-passing control member. The light receiving portion is arranged such that the first virtual extended line corresponds to the light-receiving-portion center line, when the first opening margin passes through an optical path from the light emitting portion to the light receiving portion.