Abstract: The object is to provide a reclining device for a seat such as an automotive seat including a differential transmission mechanism (Taumel mechanism) which makes it possible to eliminate precision machining of the inner surface of an external gear into which a slide bearing is pressed, in which the freedom of design of the reclining device is not restricted by the standards of a bearing bush, and which can be manufactured at a low cost. The reclining device includes a differential transmission mechanism for adjusting the opening angle of a hinge through which the seating portion and the backrest portion of the seat are coupled together. The differential transmission mechanism includes an internal gear 1 and an external gear 2 having a slightly smaller number of teeth than the internal gear and meshing with the internal gear. An arcuate gap is defined between a shaft hole 3 of the external gear 2 and a small-diameter portion of a cam 5 integral with a shaft 4 of the internal gear 1.