Yahoo Japan suspects up to 22 million user IDs may have been stolen during an unauthorised attempt to access the administrative system of its portal.
"We don't know if the file [of 22 million user IDs] was leaded or not, but we can't deny the possibility given the volume of traffic between our server and external" terminals, the company said on Friday.
The information did not include passwords and the kind of data necessary to verify a user's identity or reset passwords, it said, adding that it had updated its security measures to prevent a repeat of the incident.
Yahoo Japan is 35.5 per cent held by Japan's mobile-phone operator SoftBank and 34.7 per cent by US internet giant Yahoo. Yahoo Japan holds the country's top search-engine position with a market share of more than 50 per cent, compared with around 40 per cent for rival Google.
In 2011, Sony said information such as usernames, passwords and birthdates of more than 100 million people may have been compromised after hackers struck the PlayStation Network and Sony Online Entertainment services.
Japan acknowledges that its preventive measures against cyberattacks remain underdeveloped, with the national police agency having announced this month it would launch a team to analyse and combat them.
The Japan Aerospace Exploration Agency said last month that information related to the International Space Station may have been leaked in an unauthorised attempt to access its system.