The object of the invention is to provide a reclining device which improves engagement between a lock gear and a gear plate for reliable locking without requiring high dimensional accuracy. Four lock gears comprising a pair of first lock gears 40, and a pair of second lock gears 50 in which a gap between the second lock gear 50 and guide protrusions 12, 13 is larger than a gap between the first lock gear 40 and guide protrusions 11, 12 are adopted. Due to elastic function of a biasing member body 90a of an biasing member 90, both the second lock gears 50 are biased radially and inwardly and toward the guide protrusion 13 of the base plate 10.