A Japanese academic detained in Beijing since September has been released and returned to the country on Friday, in a move expected to help pave the way for a state visit by Chinese President Xi Jinping next year, government officials said.

The Japanese man in his 40s, a professor at Hokkaido University, was detained by Chinese authorities on suspicion of violating Chinese law, with Japanese media saying he is believed to have been accused of espionage.

He has returned to Japan and is in good health, Chief Cabinet Secretary Yoshihide Suga told a news conference.

Foreign Minister Toshimitsu Motegi declined to provide details on why the man had been detained, citing the sensitive nature of the issue, but said he and Prime Minister Shinzo Abe had been "firmly" asking for the man's release.

"We'd like to welcome President Xi as a state guest next spring in a good atmosphere and have been taking care of issues one by one," he added.

Japanese media has reported that 13 Japanese civilians have been detained in China on suspicion of engaging in spying activities since 2015.