"Until now, the ministry has taken the defence companies for granted," said Masahisa Sato, an influential ruling party lawmaker and former deputy defence minister.
Three of them, Mitsubishi Heavy, Mitsubishi Electric and IHI Corp (7013.T), which makes jet engines, bridges and heavy machinery, confirmed they had also taken part in other lower-level discussions.
Reuters asked 10 of Japan's military suppliers, including Toshiba, Mitsubishi Electric, Daikin and Subaru, for interviews with their defence unit managers.
Despite diplomatic tensions, China is Japan's top trade partner and a major manufacturing base for many Japanese companies.
Even so, Japanese companies often refer to their military products as "special equipment," the government official said.