"We don't know if the file was leaked or not, but we can't deny the possibility given the volume of traffic between our server and external" terminals, the company said in a statement late Friday.
Yahoo Japan said the data did not include passwords or the data necessary to verify a user's identity or reset passwords. The company said it has "updated its security measures" to prevent a repeat of the incident.
The company did not say exactly how the unauthorized access took place or how long the attack may have taken.
Yahoo Japan is 35.5 percent held by Japan's mobile phone operator SoftBank, and 34.7 percent held by U.S. Internet giant Yahoo! Inc.
Its popular portal Yahoo! Japan holds the top search engine position in Japan with market share of more than 50 percent.
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