The Chinese government has detained Mr. Zhu Jianrong, an expert on Chinese politics and diplomacy and a professor at Toyo Gakuen University in Nagareyama, Chiba Prefecture. This is a sign that the Chinese leadership under President Xi Jinping is trying to strengthen its control of speech to increase its ideological grip on Chinese people.

Mr. Zhu is known for media comments on issues affecting Japan and China. His detention sent shock waves among Chinese researchers and journalists in Japan and among his acquaintances, both Chinese and Japanese. The Japanese government should urge the Chinese government to release Mr. Zhu and to adopt an open-minded policy toward Chinese people who express their opinions.

Mr. Zhu went to Shanghai, his native place, on July 17. Since then, his whereabouts have not been known. A press officer of the Chinese Foreign Ministry hinted at his detention Sept. 11, saying that Chinese nationals like Mr. Zhu must abide by China’s state laws and regulations. Mr. Zhu married a Japanese researcher in 1984 and moved to Japan in 1986.

Chinese sources said that he met with Chinese military people and collected detailed military-related information for research purposes this year and that the National Security Ministry is questioning him with regard to whether his activities constituted illegal collection of information. But China has not clarifed his current situation or the specific charges against him.

Recently Mr. Zhu actively sought to improve Japan-China ties, which have deteriorated since Japan’s nationalization of three Senkaku islets in the East China Sea in September 2012.

Interchange between Japanese and Chinese researchers has served as a second track of communication between Japan and China at a time when official channels remain chilly.

The Japanese government says it cannot do anything to secure his release, because he is a Chinese national. Since Mr. Zhu played an important role in this second track of communication, the government should discard its do-nothing attitude. It should at least convey to Chinese authorities his family’s wish to meet with him and urge them to let them meet.

Separately, it also has been reported that communication was lost with Mr. Su Ling, chief editor of Xinhua Shibao, a Chinese-language newspaper published in Japan, after he went to Beijing in May. He was also active in improving Japan-China relations.

And in August, China’s public security authorities detained three Chinese well known for their mini-blogs on weibo.com, a posting website, for various suspicions.

The Chinese government has a history of persecuting people who serve as a bridge between Japan and China. In February 1998, historian Mr. Tohti Tunyaz from the Xinjiang-Uygur Autonomous Region, who was studying at a graduate school of the University of Tokyo, was detained when he returned to China to collect materials on policy toward ethnic minorities. He was imprisoned for 11 years.

According to Ajia Tsushin Co., in February 2009, Mr. Jin Xide, deputy head of the Japan research institute of the Chinese Academy of Social Sciences, was given 14 years’ imprisonment for selling Chinese state secrets to Japan and South Korea.

The truth of the allegations against both is unknown.

To firm up its political foundation, the Chinese leadership is tightening political control at home and taking a hard stance in diplomacy — when it should be pushing democratization so that China becomes an open country, domestically and internationally.

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