Confidential information from the Maritime Self-Defense Force has been leaked on the Internet via the peer-to-peer file-sharing program Winny, the MSDF said Thursday.

The leak includes personal data on dozens of MSDF members, cipher-related documents, and documents on the planning of combat exercises, the MSDF said.

The information is believed to have come from a personal computer owned by a chief petty officer in charge of communications on the destroyer Asayuki, which is deployed to the MSDF's base in Sasebo, Nagasaki Prefecture, it said.

In peer-to-peer file-sharing, unprotected data on any PC running the program can be accessed by other users by using a simple search.

The MSDF is probing why and how the sailor inputted the data into his personal computer, which he took onboard the destroyer without permission. The MSDF is also considering penalties because the sailor was not authorized to make copies of the data.

In the Defense Agency, state secrets on defense affairs are categorized into three ranks in descending order of importance -- classified, strictly confidential and confidential.

While the sailor is authorized to deal with information up to the strictly confidential category, the MSDF said its investigations have not found any leak of classified or strictly confidential data.

The leak would not affect the MSDF's use of call letters because they are changed daily, but the MSDF said it plans to revise its random-number cipher codes as soon as possible.

The computer was apparently infected with a virus that causes information disclosure from an infected computer using the file-sharing software Winny when the crew member was entering the data on the computer.

In November, personal data on about 60 people who received treatment at a Tokyo hospital run by the Ground Self-Defense Force were leaked from a doctor's privately owned PC at the hospital.