Japan has demanded China release one of its nationals after the man was arrested by Beijing on allegations of espionage.
The man, an employee of drug-maker Astellas in his 50s, has been detained since early March.
China has detained at least 17 Japanese nationals for alleged espionage since 2015, according to Kyodo News.
Relations have long been strained over the disputed East China Sea islands, wartime history and Taiwan.
On Monday, Japan’s Chief Cabinet Secretary Hirokazu Matsuno said China had informed the country of the arrest through diplomatic channels and requested a consular visit.
But he did not disclose any details about the man’s identity or his alleged offence.
Chinese Foreign Ministry Spokesperson Mao Ning said later in the day that the man was being held for espionage.
“China will handle the case in accordance with law and will protect the rights of the person in accordance with law,” she said.
Ms Mao said Japan needed to do more to ask its citizens to refrain from engaging in such activities.
Astellas Pharma confirmed that the man was its employee but refused to divulge his identity.
In 2019, China detained a Japanese professor who specialises in modern Chinese history on suspicion of spying. He was released after two months.
China-Japan relations have shown no signs of improvement in recent years.
In February, the two countries held a security dialogue in Tokyo for the first time in four years. During which, Japan criticised China’s ties with Russia and its alleged use of spy balloons while Beijing expressed concern over Japan’s military build-up.
