A systems engineer admitted Tuesday to copying information about millions of customers from the computer servers of education service provider Benesse Corp., but he told a court he did not know the data was confidential.

Masaomi Matsuzaki, 39, was appearing on the first day of his trial at the Tachikawa Branch of the Tokyo District Court.

In their opening statement, prosecutors said Matsuzaki stole the customer data to sell to traders of lists of names and personal details, in order to pay off gambling debts.

His lawyers reserved judgment on the prosecutors' statement.

According to the indictment, Matsuzaki illegally downloaded and copied on two occasions, on June 17 and June 27, the personal data of around 30 million Benesse customers onto his smartphone from the Tokyo branch of a database management contractor for Synform.

Of the 30 million customers, details of 10 million were sold to other firms, the prosecutors allege.

The data included the names and addresses of children and their birth dates. Matsuzaki was employed as a temporary worker at the contractor at the time.

Matsuzaki downloaded the data from Benesse's database and copied it by unlocking its leak prevention system, according to investigators. They also found that at least 10 name-list brokers ended up with the data.