The present invention provides a rotation detecting device having a small size, low manufacturing costs, and excellent quietness, and an automobile equipped with the rotation detecting device. The rotation detecting device comprises a non-rotating internal gear 2b concentrically arranged with the rotation center of a rotary body (a steering shaft), a planetary gear 4 that meshes with the internal gear 2b and that rotates and revolves on the inner circumference of the internal gear 2b with the rotation of the rotary body, a rotation transmitting member 6 that is concentrically arranged with the rotation center of the rotary body and that rotates on its own axis by the rotation of the planetary gear 4, a scale plate 7 provided in the rotation transmitting member 6, and a rotation angle detecting unit 11 arranged to face the scale plate 7 for detecting the rotation angle of one or more full rotations of the rotary body.