Abstract: A locking device of a seat back of a trunk through-seat in order to prevent theft from a trunk compartment by maintaining the locking state of the seat back with the aid of key operation. The locking device includes lock pins which close or open the seat back stoppers which limit the movement of the lock pins in an upward direction, levers which move the lock pins in an upward direction by rotating the lock pins with the aid of the operative members, and keys which execute the slide of the stoppers. The stopper is moved laterally from a position above the lock pin and then the lock pin is moved in the upward direction by rotating the lever. Thus the lock of the seat back is released.