A classified Maritime Self-Defense Force radio communications manual has disappeared from a destroyer based in Hiroshima Prefecture, officials said Thursday.
Officials said the MSDF is investigating the case and may mete out punishment.
The missing document from the destroyer Aokumo, based at the Kure District Headquarters and used for training missions, outlines communication procedures between vessels and planes during military operations and is classified as "secret," the Defense Agency's third-highest level of secrecy, the officials said.
It reportedly also includes data on radio communications with U.S. forces, although the MSDF officially refuses to confirm this.
The document is usually kept locked in the vessel's combat information center, which is manned by around 20 service members.
"It is a very important document," a senior Defense Agency official said.
Last month, data on computer systems used at the Air and Ground Self-Defense Forces were found to have been leaked to outsiders from Fujitsu Ltd., which was in charge of creating the system on contract with the agency.