A monitor showing the logo of Mitsubishi Materials Corp is seen in Tokyo, Japan, November 24, 2017. REUTERS/Toru Hanai

TOKYO (Reuters) - Japan's Mitsubishi Materials Corp 5711.T said on Friday it has found fresh cases of misconduct at its Naoshima copper smelter and refinery in western Japan and the plant's Japanese Industrial Standards (JIS) certification has been revoked.

The finding follows its announcement in May that it had found new cases of improper actions at a small number of its group firms, on top of the five units that had already confirmed falsification of data. The new cases have widened the number of affected customers from 762.

It is the latest blow to the diversified materials manufacturer, which first admitted late last year that its subsidiaries falsified data about products, including parts for aircraft and automobiles, one more in a series of quality assurance scandals to rock Japanese manufacturers.

Japan’s Ministry of Economy, Trade and Industry separately said the Quality Assurance Organization revoked the JIS certification for the Naoshima plant’s copper slag products - used in concrete construction materials - because it had shipped products with a JIS label when they did not meet the criteria or did not conduct required tests.

A total of five customers have received those slag products and Mitsubishi Materials is making safety checks with these customers, a company spokesman said.