A Maritime Self-Defense Force plane on Wednesday sighted a surfaced Chinese submarine off Kagoshima Prefecture, MSDF officials said.
It is the first time that a Chinese submarine has been spotted on the surface in waters around Japan, they said.
The Ming-class sub was flying a Chinese flag, they said.
A P3-C surveillance plane sighted the submarine at around 8 a.m. heading west in the Pacific some 40 km east of Cape Sata, they said.
The submarine later went through the Osumi Strait between the cape and the Osumi Islands, which includes Tanegashima and Yakushima.
The center channel of all Japanese straits are considered open seas.
At around 5 p.m., it was traveling on the surface heading west in the East China Sea, some 120 km southwest of the Satsuma Peninsula, according to the officials.