A recliner adjuster includes a guide bracket to be fixed to one of a seat cushion and a seat back that can pivot with respect to the seat cushion, an internal gear to be fixed to the other of the seat cushion and the seat back, a cam interposed between the guide bracket and the internal gear, a pair of lock gears radially slidable in association with an operation of the cam, and a mounting ring for assembling the guide bracket and the internal gear together with the cam and the pair of lock gears interposed therebetween. The mounting ring has a flat annular portion and a cylindrical side portion joined to one of the guide bracket and the internal gear. An outer peripheral surface of the other of the guide bracket and the internal gear is held in sliding contact with an inner surface of the cylindrical side portion of the mounting ring to thereby radially position the other of the guide bracket and the internal gear.