Japanese Prime Minister Fumio Kishida described it as a "possible ballistic missile launch".
"It is truly regrettable that North Korea has continuously launched missiles since last year," he told reporters.
Kishida said the Japanese government was analysing details, including how many missiles may have been launched.
"There have been no reports of damage to Japanese aircraft and vessels so far," Japan's top government spokesman Hirokazu Matsuno told reporters.
"We are continuing analysis, but if it took a normal orbit, it is expected to have travelled about 500 kilometres and fallen outside Japan's exclusive economic zone."
The launch followed a speech by Kim last week in which he said North Korea would continue to build up its military capabilities.
"I expect North Korea to continue refining its arsenal as a way to improve its strategic position at a time of political change in the region," Jean Lee, a senior fellow at the Washington-based Woodrow Wilson International Center, told AFP.
In 2021, North Korea said it successfully tested a new type of submarine-launched ballistic missile, a long-range cruise missile, a train-launched weapon, and what it described as a hypersonic warhead.
The dialogue between Washington and Pyongyang remains stalled, following the collapse of talks between Kim and then-president Donald Trump in 2019.
Under Trump's successor Joe Biden, the United States has repeatedly declared its willingness to meet North Korean representatives, while saying it will seek denuclearisation.
But Pyongyang has so far dismissed the offer, accusing Washington of pursuing "hostile" policies.
At the end of a key meeting of the ruling Workers' Party last week, he did not mention the United States at all.
Instead of the policy positions on diplomacy for which Kim's New Year statements have been closely watched in recent years, he focused on food security and development in an extensive speech.
But he said Pyongyang would continue to boost its capabilities keeping in mind "the military environment of the Korean peninsula" and the changing international situation.
"Pyongyang is sending the message to the US that it will not change and therefore Washington must give in," Shin Beom-chul, a researcher at the Korea Research Institute for National Strategy, told AFP.
North Korea is under multiple sets of international sanctions over its nuclear and ballistic missile programmes.
The impoverished nation has also been under a rigid self-imposed coronavirus blockade that has hammered its economy.
The worsening economic situation during the pandemic, however, has not blunted those programmes, and North Korea has continued to pursue weapons development, a UN report said in October.
Concerns have grown about a full-blown food crisis in North Korea, and a United Nations human rights expert warned in October that the most vulnerable were "at risk of starvation".