A rotation detecting unit is adapted for a rotatable body. The rotation detecting unit includes a rotation detecting part, a signal transmission member, and a main body part. The rotation detecting part detects a rotational state of the rotatable body and outputs a rotation detection signal. The signal transmission member is electrically connected to the rotation detecting part to transmit the rotation detection signal to an external device. The main body part holds at least a part of the signal transmission member and the rotation detecting part. The main body part is integrally molded from thermosetting resin including a joined part obtained as a result of joining together a terminal of the rotation detecting part and the signal transmission member, the part of the signal transmission member, and the rotation detecting part. The main body part includes a recess at a region corresponding to the rotation detecting part.