Sony's share price has tumbled following the hacking attacks on its TV and film unit, and its European subsidiary's website.
The breaches undermined confidence in Sony and sent its shares down more than 2% by midday Monday in Tokyo trade, according to AFP reports.
Sony said although its European subsidiary's website had been accessed illegally, no confidential data was stolen or published on the Internet.
"All the data that the hacker seems to have copied was information already available on the company website," said company spokeswoman Mami Imada in a statement issued in Tokyo.
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Late on Friday, Sony Pictures Entertainment confirmed that some of its websites were breached just as it restored its PlayStation Network and online entertainment services after the personal data of up to 100 million users was stolen in April in one of the largest data breaches to date.
Sony has estimated the data breach will result in a $170m (£104m) hit to its operating profit. Pundits say the cost of reputational damage is likely to be much greater.
Confirmation of the intrusion came several hours after hacker group Lulz Security (LulzSec) claimed to have accessed the servers hosting Sony Pictures Entertainment and obtained the e-mail addresses, birth dates and passwords of more than one million users.
LulzSec claims to have hacked into an unencrypted database using the well-known SQL injection attack method. "This is disgraceful and insecure: they were asking for it," a statement from Lulz Security said.
Last month, Sony chief executive Howard Stringer said Sony was conducting a major review of online vulnerabilities.
Read more about the Sony hacks: