Japan's Maritime Self-Defense Force says a captain's inadequate navigational safety examination caused the destroyer Inazuma to run aground in January.
The MSDF's internal investigation committee had been looking into the accident, which occurred during trial navigation in waters off Yamaguchi Prefecture.
In the investigation results released on Tuesday, the committee says the trial run was ahead of schedule and the captain issued an order to change the ship's route.
It says the captain failed to take safety into consideration, and that the vessel was traveling at its maximum speed of roughly 55 kilometers per hour. It calls the captain's supervision inappropriate.
The MSDF says a senior person in charge of navigation who was on the bridge failed to check nautical charts for obstacles along the course when the captain's order was issued.
It says the person did not report the plan to alter the course to the ship's command post, which was watching surrounding areas.
It attributes these actions to the person's inadequate knowledge and safety awareness.
The investigation report also says command post crew notified the bridge multiple times about obstacles on the route, but that the information did not reach the senior person on the bridge.
The committee says these factors led to the accident.
To prevent a recurrence, the committee says training for captains should be reconsidered. It also says navigational safety studies should be enhanced and more communication methods put in place so that safety information will be properly conveyed to senior crew members.
The MSDF says repair work for the destroyer is expected to cost roughly four billion yen, or nearly 30 million dollars, and take several years. They say they will take the findings of the committee into account and consider disciplinary measures for relevant personnel.
MSDF Chief of Staff Admiral Sakai Ryo said on Tuesday that he is embarrassed that the vessel caused the accident by neglecting basic safety rules of navigation.
Sakai said the MSDF will strengthen safety measures based on the investigation results and work to prevent a recurrence.
The admiral said he takes seriously that the destroyer will be out of service for repairs that are expected to take several years. He added that other vessels, including destroyers, will cover the Inazuma's tasks.
The Japan Coast Guard has been conducting its own investigation of the accident on suspicion of professional negligence endangering maritime traffic.