The Japanese government said it was investigating whether the data included sensitive information on nuclear materials.
Mitsubishi Electric Industrial Co. said the information -- 44 megabytes of files containing inspection forms, reports and manuals used from 2003 to this year -- probably appeared on the Net sometime after March, but company officials weren't aware of the leak until Wednesday.
The files from Tokyo-based affiliate Mitsubishi Plant Engineering Corp. had been saved on a worker's personal computer, which was loaded with the file-sharing software Winny, said a company spokesman, who declined to be identified. A virus that infected Winny sent those files to the Net, he said.
"We haven't determined whether there was a leak of critically important information relating to the security of nuclear materials," Japan's Chief Cabinet Secretary Hiroyuki Hosoda told reporters.
A Mitsubishi Electric spokesman, who declined to be identified, said the information was from seven Japanese electric power companies and four other utility industry firms. It included photos of power generation facilities and workers' medical files -- data that should not have been loaded onto a personal computer, he said.
Reports said the leak was mainly reports from inspections at two nuclear reactors run by Kansai Electric Power Co., and plants operated by Hokkaido Electric Power Co. and Kyushu Electric Power Co.
Though confidential, the data didn't appear to include anything about nuclear materials, according to media reports.
The Mitsubishi Electric spokesman said the company had submitted a formal report to the Economy, Trade and Industry Ministry and was taking steps to prevent a recurrence.
Winny was developed by a Japanese programmer in 2002 and has been illegally used to trade copyright material, such as movies and game software, online.
