Islamic State (IS) militants have posted a video online purporting to show two Japanese hostages and demanding "$200 million" from the Japanese government to save their lives.
It features a knife-wielding black-clad figure, standing in a desert area along with two men wearing orange clothing.
The person criticised what he called Japan's support for a US-led coalition waging a military campaign against IS in Iraq and Syria.
Japan's government said it was looking into the threat.
"We are aware of the reports. We are in discussions on the matter," said an official in the foreign ministry's terrorism prevention division, declining to be named.
When asked whether the government regarded the video as authentic, he told AFP: "We are checking that too."