To provide a rotational driving force transmitting device that can use cost-effective magnets for transmitting rotational driving force, and that can easily change reduction ratio, and to provide a roller conveyor device having this rotational driving force transmitting device.

In the rotational driving force transmitting device 30, a disk type driven magnet 60 is fixed at the end of a driven shaft 20. In this driven magnet 60, a plurality of split magnetized regions 61a, 61b, 61c, etc. provided with S poles and N poles in the thickness direction are formed at the intervals of an equal angle in the circumferential direction, and in such a way that different magnetic poles are positioned alternately. Disc type driving magnets 70 are fixed to a driving shaft 40. In each driving magnet 70, a plurality of split magnetized regions 71a, 71b, 71c, etc. provided with S poles and N poles in the thickness direction are formed at the intervals of an equal angle in the circumferential direction, and in such a way that different poles are positioned alternately.