SHANGHAI (Kyodo) -- A Chinese professor at a university in Kobe, western Japan, who went missing after visiting his country in the summer of 2023 returned to Japan in January, the college said Friday, with the reason for his disappearance remaining unknown.

Before his return to Japan on Jan. 24, speculation was rife that Chinese authorities had detained Hu Shiyun, a professor at Kobe Gakuin University, for suspected espionage.

An expert familiar with Sino-Japanese relations said there were signs of improvement in ties in January and that this might have played a role in bringing about Hu's return to Japan.

Hu was in charge of Chinese-language courses at the university's faculty of global communications and formerly headed the faculty. The college said it has yet to be decided whether he will resume work at the school.

Hu became unreachable after arriving in China in August 2023 to conduct research. His family told the university in late September that year he could not return to Japan "due to illness" and informed it in October that they had not heard from him.

Hu's disappearance came at a time when China was detaining a number of people, including foreign nationals, on suspicion of engaging in spying activities.

In 2019, Yuan Keqin, a Chinese professor at Hokkaido University of Education, was detained by Chinese authorities on suspicion of espionage. He was sentenced to six years in prison in January 2024, according to a source familiar with the matter.

Fan Yuntao, a professor at Japan's Asia University, has been unreachable in China since February 2023, sources familiar with bilateral relations said in April last year.