About 1.7 million electric devices containing toxic polychlorinated biphenyl are being kept at factories nationwide, according to officials of the environment and industry ministries.
Production of the substance has been banned since 1972.
The devices, including high-voltage electric transformers, are stored at such facilities as steel, rolling stock and automobile plants, according to officials of the Environment Ministry and the Ministry of Economy, Trade and Industry.
The devices are not harmful to the environment because they are sealed, they said.
The ministries estimated the number of PCB-containing devices from manufacturers' records, the officials said.
Despite the 1972 production ban, the devices were still being manufactured until around 1990, they said.
The ministries' findings might prompt the government to revise its plan to complete detoxifying and disposing of PCB-containing materials by March 2016.
PCB was used in insulation oil for such devices as transformers and capacitors.
In 2000, it was discovered that transformers being produced by some manufacturers still contained PCB, prompting the then Ministry of International Trade and Industry to ask the Japan Electrical Manufacturers' Association to investigate.
The association found that the transformers had been contaminated in the production process; manufacturers had used insulation oil made from old oil that contained PCB. It said production of the PCB-contaminated insulation oil has been halted since 1990.
The ministries will report the results of the investigations to a joint meeting on PCB-containing products.