P&G, known for products such as Tide detergent and Pampers diapers, had said in February it was looking to unload its Attento diaper brand because it did not fit into its strategy of focusing on products that it can expand on a global scale.
Daio and P&G did not disclose financial terms.
P&G launched the Attento brand nationwide in 1982. Its diapers are now sold mainly in Japan, with some sold in Hong Kong. P&G does not sell adult diapers anywhere else.
The Japanese market for paper diapers is worth about 60 billion yen ($489 million).
Nomura Securities, a unit of Nomura Holdings Inc. 8604.T, handled the deal.
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