The man in the video did not give any information about who was holding him or any demands they had.
A video surfaced on Thursday appearing to show a missing Japanese journalist, reportedly held by an al Qaeda affiliate in Syria, reading a message to his family and country.
"Hello, I am Jumpei Yasuda. Today is my birthday, 16 March," the bearded man said on the video, posted on Facebook.
The man, sitting at a table in front of a white wall, said he missed his family but could not be with them. The man, who was dressed in a dark sweater with a scarf, mostly seemed calm as he spoke in English in the one-minute video but occasionally paused with emotion.
Japanese media said Yasuda was captured by Nusra Front after entering Syria from Turkey in June. Public broadcaster NHK said it had spoken by phone with the man who posted the video, who said he had received it from someone seeking Yasuda's release.
Chief Cabinet Secretary Yoshihide Suga said the man in the video appeared to be Yasuda.
"The safety of Japanese citizens is one of our most important duties and we are gathering information and making all possible efforts to respond," Suga told a news conference, declining to give details.
He added that the government was not aware that any ransom request had been made.
The Islamic State militant group beheaded two Japanese nationals - a self-styled security consultant and a veteran war reporter - early last year. The gruesome executions captured the attention of Japan but the government said at the time it would not negotiate with the militants for their release.
Prime Minister Shinzo Abe came under fire for his handling of hostage cases, something he would like to avoid with an election looming this summer. Critics say his more robust security stance risks getting Japan involved in more international conflicts.
Reuters could not verify the video or reach Yasuda's family, but Kyodo news agency quoted his mother as tearfully saying: "I just hope he comes home safely."
The man in the video did not give any information about who was holding him or any demands they had. He said "they" allowed him to say what he wanted.
After saying he wished he could hug his wife, father, mother and brother, he said: "I have to say to something to my country: When you're sitting there, wherever you are, in a dark room, suffering with the pain, there's still no one. No one answering. No one responding. You're invisible."
In December, media freedom organisation Reporters Without Borders retracted and apologised for a report it had issued that said Yasuda had been threatened with execution in Syria. The government said at the time it was seeking information.