To provide a reclining device that ensures locking strength even when it is hard hit.

The reclining device 10 includes: a base plate 11 fixed to a sheet cushion 21 or a seat back 22; a gear plate 12 assembled to the base plate 11 and having an internal tooth; fixing guides 13 formed at the base plate 11; four lock gears 14 sliding between the fixing guides 13 and have external teeth capable of meshing with the internal tooth of the gear plate 12; movable guides 15 intervening between the lock gears 14 and the fixing guides 13; a cam 16 for causing the lock gear 14 to mesh with the gear plate 12; a link plate 18 for rotating together with the cam 16 to release the mesh of the lock gear 14 with the same; and a torsion spring 19 for urging the movable guides 15, wherein two lock gears 14 are caused to mesh with the internal tooth of the gear plate 12 within a range of 40 to 50 degrees with respect to a torso angle 24 of the seat back 22.