JAL flight JL636, a Boeing 767-300, was radar vectoring to approach Narita Airport(RJAA) for runway 36R, it encountered severe turbulence at 9,500ft, 40nm SE of HKE (Hokusoh) VOR/DME, and one passenger(female) in its aft cabin has been thrown out of her seat. PROBABLE CAUSE It is estimated that this accident occurred as follows: while passing through the frontal zone generated north of a typhoon, the aircraft experienced powerful turbulence as it encountered intense air disturbance, upon which the seatbelt of one passenger came unfastened, causing the passenger to fly up from her seat and then hit her body against the back rest of the seat in front, resulting in a serious injury. With respect to the unfastening of the seatbelt, when the aircraft experienced the severe turbulence, it is estimated that the bag the passenger was holding on her lap touched the buckle of the seatbelt, causing the flap to be pulled upward