BEIJING--A Hokkaido University professor who visited China in September is being detained here by authorities, sources close to Japan-China relations said on Oct. 18.

This is the first case of a Japanese national university professor, who is deemed a quasi-public servant, being detained in China.

Why the professor, who is in his 40s, is being held is not clear. However, it is possible that he is under suspicion of violating China's anti-spying law or criminal law.

The professor, specializing in Chinese politics, worked for the National Institute for Defense Studies, an arm of Japan’s Defense Ministry, as well as the Foreign Ministry in the past.

In recent years, security has been tightened against information-gathering activities in China by foreigners.

In 2014, China enacted the anti-spying law, which defines spying activities, and in 2015 it implemented a national security law to crack down on intelligence activities.

Since 2015, 13 Japanese men and women have been detained on suspicion that they were engaging in spying activities. They were all civilians.