This coupling device includes a magnet rotator and a yoke-side member, while a conductor portion of the yoke-side member has a plurality of first conductor portions at least on a side opposed to magnets. A yoke of the yoke-side member is arranged at least on the side opposed to the magnets in a clearance between the plurality of first conductor portions. The ratio of the circumferential length of each of the first conductor portions to the circumferential length of the yoke is at least 1/1.4.