A seat reclining device applied to a seat device in which a seat belt is fixed to a seat back at one side comprising: a pair of upper arms (54a, 54b) mounted on the seat backs (12a, 12b); a pair of lower arms (57a, 57b) fixed to seat cushion (16a, 16b) at both sides so as to be rotatable relative to the upper arms (54a, 54b); a pair of locking mechanisms (60a, 60b) restricting a rotation of the upper arms (54a, 54b) relative to the lower arms (57a, 57b); a connecting mechanisms unlocking both locking mechanisms (60a, 60b); and an operation handle provided at one locking mechanism; the other locking mechanism is unlocked in response to operation of the operation handle through the connecting mechanism; the seat back includes seat back frames (12a, 12b) provided at the right and left sides of the seat back; the one locking member having the operation handle is positioned on the side of the one seat back frame having the belt supporting member; the seat reclining device being characterized in that the seat belt extends from a retractor and is guided to the front side of the seat device via a belt supporting member provided at the upper end of one seat back frame, wherein under the condition that the operation handle is operated so that one locking mechanism can be unlocked, one locking mechanism, the operation handle, the connecting mechanism and the other locking mechanism can be engaged with each other and operate together, under the condition that the one seat back fixed to the seat belt is more deformed by tension of the seat belt than the other side of the seat back, any engagement among the one locking mechanism, the operation handle, the connecting mechanism and the other locking mechanism is disengaged from each other so that the one locking mechanism cannot operate together with the other locking mechanism, and wherein a shaft included in the locking mechanism is provided with a convex portion and a groove portion is provided at the connecting mechanism where the shaft is inserted, or convex portion is provided at the connecting mechanism where the shaft is inserted and the shaft included in the locking mechanism is provided with the groove portion so that predetermined interspace is set between the convex portion and the groove portion. a pair of upper arms (54a, 54b) mounted on the seat backs (12a, 12b); a pair of lower arms (57a, 57b) fixed to seat cushion (16a, 16b) at both sides so as to be rotatable relative to the upper arms (54a, 54b); a pair of locking mechanisms (60a, 60b) restricting a rotation of the upper arms (54a, 54b) relative to the lower arms (57a, 57b); a connecting mechanisms unlocking both locking mechanisms (60a, 60b); and an operation handle provided at one locking mechanism; the other locking mechanism is unlocked in response to operation of the operation handle through the connecting mechanism; the seat back includes seat back frames (12a, 12b) provided at the right and left sides of the seat back; the one locking member having the operation handle is positioned on the side of the one seat back frame having the belt supporting member; the seat reclining device being characterized in that the seat belt extends from a retractor and is guided to the front side of the seat device via a belt supporting member provided at the upper end of one seat back frame, wherein under the condition that the operation handle is operated so that one locking mechanism can be unlocked, one locking mechanism, the operation handle, the connecting mechanism and the other locking mechanism can be engaged with each other and operate together, under the condition that the one seat back fixed to the seat belt is more deformed by tension of the seat belt than the other side of the seat back, any engagement among the one locking mechanism, the operation handle, the connecting mechanism and the other locking mechanism is disengaged from each other so that the one locking mechanism cannot operate together with the other locking mechanism, and wherein a shaft included in the locking mechanism is provided with a convex portion and a groove portion is provided at the connecting mechanism where the shaft is inserted, or convex portion is provided at the connecting mechanism where the shaft is inserted and the shaft included in the locking mechanism is provided with the groove portion so that predetermined interspace is set between the convex portion and the groove portion.