The case has strained relations between Seoul and Tokyo, which condemned the extension as a "grave humanitarian issue" and said it would issue a formal protest.
"I was told by my lawyers that the ban on my leaving the country had been renewed for three more months," Tatsuya Kato told AFP.
The travel ban on the former Seoul bureau chief of Japan's conservative Sankei Shimbun has been in force since August last year.
The charge stems from an August article he wrote about Park's whereabouts on the day the Sewol passenger ferry sank in April with the loss of around 300 lives.
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At a preliminary court hearing in November, the journalist insisted he had no intention of defaming Park.
The next hearing is scheduled for Monday.
Media freedom group Reporters Without Borders has criticised the trial, arguing that Kato's report was clearly in the public interest.
South Korean defamation law focuses on whether what was said or written was in the public interest rather than whether it was true.
"The (Japanese) government will convey our concern to the South Korean side and urge them to take appropriate action," Chief Cabinet Secretary Yoshihide Suga told reporters in Tokyo.
"It's a grave humanitarian issue," Suga added.
Seoul's foreign ministry dismissed the criticism and urged Tokyo to remain "cool-headed" over a case that it said was being treated in accordance with legal norms.
"This has nothing to do with bilateral relations and it is not desirable that Japan is trying to escalate this into a diplomatic row," ministry spokesman Noh Kwang-Il told reporters.