Mitsubishi Materials says one of its subsidiaries, Mitsubishi Aluminum, fabricated inspection data for aluminum plates so they appeared to meet quality standards.
The plates were manufactured at the metal maker’s factory in Shizuoka Prefecture.
Mitsubishi Materials acknowledged last week that 2 other group firms, Mitsubishi Cable Industries and Mitsubishi Shindoh, delivered sub-standard products with altered data.
But it did not disclose the misconduct at the aluminum unit, which was actually discovered during an in-house probe in November last year.
Mitsubishi Aluminum says the sub-quality products were delivered to 16 clients, but it has confirmed that the products have no safety problems.
Affected products from the 3 subsidiaries have been supplied to 274 clients.