China has freed three of four Japanese nationals detained a week ago and accused of intruding into a restricted military area in Hebei.
The Xinhua news agency said the fourth man was still being investigated for allegedly filming military sites.
They were held amid a diplomatic row between Beijing and Tokyo.
The dispute between the two neighbours centres on the seizing by Japan of a Chinese fishing boat and its crew after a collision in disputed waters.
Kyodo news agency in Japan reported that the four involved were in the construction industry and were believed to have been preparing to bid on a project to dispose of chemical weapons abandoned by Japanese forces in China at the end of World War II.
But they were accused of entering a closed military zone and arrested. According to state media the three Japanese nationals released on Thursday have admitted violating Chinese law.
The BBC's Michael Bristow, in Beijing, says the release should help ease tensions between China and Japan.
Tensions have been high since Japan detained a Chinese captain whose fishing boat collided with Japanese patrol boats earlier this month.
The trawler was sailing in an area claimed by both countries, near uninhabited islands which may have oil and gas deposits.
Japanese prosecutors suspected the captain may have rammed the patrol ships deliberately.
He was eventually released after two weeks, but Japan has demanded an apology and insisted that China must pay damages for the repair costs of its two damaged patrol vessels.
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