The man in his 40s has been held since September in Beijing on unspecified charges of a possible violation of Chinese law, the chief Japanese Cabinet secretary, Yoshihide Suga, told reporters in Tokyo.
Suga gave no details of the charges, but the Kyodo News agency cited unidentified sources as saying the man may be accused of spying. It said the man was a professor at Hokkaido University and a former Japanese Defense Ministry researcher.
A Chinese foreign ministry spokeswoman, Hua Chunying, said she had no information on the case. She expressed hope Japan's government would advise its citizens to "respect Chinese laws and regulations."
The Asahi newspaper, citing unidentified sources, said the man entered China on Sept. 3 and was arrested at the Beijing airport as he was leaving following a two-week visit.