The Yokohama Municipal Government mistakenly alerted its Twitter followers Wednesday morning that North Korea had launched a missile, only to apologize 20 minutes later for the error.
The city's crisis-management office sent out the alert around 11 a.m.
Tensions with the North continue to escalate following a litany of nuclear threats from the communist regime.
The office prepared a warning statement in advance to immediately respond to any such emergencies, according to a city official.
But someone in the municipal government apparently slipped up and sent out the tweet, the official said.
A call from an upset Yokohama citizen soon came in pointing out the mistake, and the agency hastily issued an apology on its account.
"We will further look into why this kind of thing happened, and will make sure there will be no recurrence," the official said.
The crisis-management agency currently has about 40,000 followers on Twitter.