A Viking Air DHC-6-400 operated by First Flying (Dai-ichi Koku) suffered substantial damage in a runway excursion accident on landing. The Twin Otter took off from Naha, Okinawa at 08:38 hours with 14 people onboard. The copilot was Pilot Flying. After a short flight, an approach was flown to runway 19 at Aguni Airport. After touchdown it veered to the right, went off the runway and impacted a fence and trees. The right main landing gear was twisted, and the nose section of the fuselage and the propeller of the No.2 engine were also damaged, but there was no fire. Eleven occupants were injured. It appeared that the nosewheel centering lock had disengaged during the flight, which went unnoticed by the crew. This particular plane was delivered to DAK on 31 March 2015, and was in service on 2 August 2015. DAK has another DHC-6-400, JA202D. Aguni Airport has a single runway 01/19 with 800 m x 25 m dimension. The weather data of Aguni Airport at the time of the accident by AMeDAS data: 09:00JST, wind southeast 3.5 m/s (7 kt), no precipitation, temperature 27.3 degree-Celsius. Probable Causes: It is highly probable that this accident occurred because, when the Aircraft landed, the PF could not properly control the aircraft that had started to deflect after touchdown, as a result of which it departed from the side of the runway and collided with the Perimeter Fence, and the aircraft was damaged. It is probable that the Aircraft started to deflect after touchdown because the PF forgot the checklist and the PIC as PM did not properly monitor or did not perform the necessary pointed out, as a result of which the Aircraft touched down with the nose wheel deflected to the right. It is somewhat likely that the PF could not properly control the Aircraft that had started to deflect after touchdown because he did not have sufficient knowledge concerning the aircraft system of the Aircraft, as a result of which he could not fully understand the situation when the deflection started. It is somewhat likely that the inadequate response of the PIC in the event of an unforeseen situation contributed to this. It is probable that PF had insufficient knowledge and could not fully understand the situation when the deflection started, because the Company had not properly confirmed the effectiveness of ground school training that should be held prior to route training and training on the establishment of knowledge.