A chemical product unit of Hitachi Ltd. said Friday it has confirmed improper inspections of its auto parts, display materials and other products, in another revelation of unreliable quality testing in the Japanese manufacturing industry.

Hitachi Chemical Co. said the number of companies that contracted to purchase the products in question had risen to 2,400 from the 500 it had disclosed in June, when it admitted to faulty inspections of industrial batteries. It also revealed improper testing of materials to protect semiconductors on Monday.

"I offer a deep apology for causing concern to related parties," President and Chief Executive Officer Hisashi Maruyama said at a news conference in Tokyo. He said he would not step down to take responsibility for the time being, citing an ongoing investigation on the improper inspections.