The incident took place near the island of Yakushima, the news agency said, without providing any other details.
The aircraft disappeared from radar at 14:40 local time, and the Maritime Safety Agency received a report of the crash five minutes later.
Over the years, there have been many incidents involving the Osprey tiltrotor military aircraft. In 2012, two Ospreys crashed in the US state of Florida and in Morocco; in 2016, an incident occurred in the Persian Gulf area: a Marine died. In August 2017, an aircraft crashed off the east coast of Australia.