The leaked information does not include passwords and data necessary for identity verification to reset passwords, the agency said.
The company detected the access attempt at around 9 pm on May 16, 2013, Kyodo reported.
Yahoo Japan checked its system after severing access and found traces of an attempt to steal user IDs, according to Kyodo.
Yahoo Inc has a 35 percent stake in Yahoo Japan.
© Thomson Reuters 2013
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