Mitsubishi factories building guided missiles, submarines and components for nuclear power plants have been hit by hackers, according to local reports.
According to the Yomiuri newspaper, Mitsubishi Heavy Industries - Japan's largest defence contractor - was hit in August, with about 80 computers at the company's Tokyo headquarters found to be infected with viruses.
Several of the company's R&D and manufacturing sites were also attacked. These include the Kobe Shipyard, which builds submarines and makes components to build nuclear power stations, the Nagasaki Shipyard, which builds escort ships, and the Nagoya Guidance & Propulsion System Works, which makes guided missiles and rocket engines.
At least eight different viruses were found, including at least one Trojan - although it's not known whether any data was stolen.
The attack is the first yet reported against Japan's defense industry - although the country's defense department warned only last month that such attacks could be on the cards.
Nobody's yet pointing the finger at a possible culprit, although the extent of the breach implies it's a state rather than an individual.
China, which has been blamed for attacks earlier this year against Lockheed Martin and other US defense contractors, is likely to come under suspicion.
Mitsubishi says it'll release a statement later today when it's carried out further investigations.