A Jin Air Boeing 737-800, registered HL8243, took off from Kitakyushu Airport, and during the climb to the cruising altitude bound for Incheon International Airport in the Republic of Korea, the aircraft experienced turbulence, which caused a cabin crew member to fall down resulting in her injury. In the cockpit, the PIC was in the left pilot?s seat as Pilot Monitoring, a trainee in the FO promotion training in the right pilot?s seat as Pilot Flying and the FO was in the observer seat. When reaching an altitude of 10,000 ft during the climb, the flight crew members confirmed that there were no clouds on the climb route, and turned off the seat belt sign in the cabin around 13:12 JST. After that, the aircraft continued to climb to the cruising altitude of FL320. After the seat belt sign was turned off, based on the weather information confirmed prior to their departure, the PIC anticipated the possibility that they might encounter turbulence during the climb, thus he was thinking of how to respond, such as promptly turning on the seat belt sign to have the cabin crew members seated, when detecting a sign of turbulence. On the other hand, in the cabin, after the seat belt sign was turned off, cabin crew members started selling duty-free goods and distributing immigration cards. The senior cabin crew member moved to the aft galley to take care of passengers. Until reaching an altitude of about FL 250 after the seat belt sign was turned off, the aircraft had neither entered clouds nor encountered turbulence, and any sign of turbulence was not confirmed on the airborne weather radar and other instrument indications. However, around 13:17 immediately after reaching FL 250, the aircraft was strongly shaken all of sudden, therefore the PIC turned on the seat belt sign at once. At this time, the PIC instructed the cabin crew members to be seated promptly. When this shaking occurred, the purser in the center of the aft galley was standing facing forward of the cabin, but fell down to the right not being able to support herself. Afterward, being seated in her seat located near the aft galley, the purser confirmed that she had pain in her right ankle and suffered abrasions. Turbulence ceased after reaching FL 270, and the flight reported to Fukuoka Air Traffic Control that they encountered turbulence. The PIC then judged that it would be possible to fly to Incheon International Airport, their destination. The purser was diagnosed with a lateral malleolus fracture in her right ankle in a hospital in Seoul City.