The Defense Ministry lodged a protest with Beijing over the intrusion of a Chinese naval survey ship into Japanese territorial waters near Kyushu on Feb. 12.
The ministry announced that it expressed “grave concern” to Chinese authorities through diplomatic channels.
This marked the seventh such incident since 2021, with the last occurring in December 2022.
Ministry officials have been on the alert regarding the increasingly frequent appearances, every month or two, of Chinese survey vessels in the area.
The survey ship trespassed into Japan’s sovereign waters in the early hours of Feb. 12 off the coast of Yakushima island in Kagoshima Prefecture, south of the main southern island of Kyushu, according to the ministry.
The Maritime Self-Defense Force dispatched patrol aircraft and a missile-equipped vessel to monitor the Chinese ship.
Ministry officials spotted the vessel sailing north toward Yakushima before it entered territorial waters off the island at around 2:30 a.m.
The vessel left Japanese waters at around 4:10 a.m., when it sailed westward off Kuchinoshima, a smaller island southwest of Yakushima.
The ministry has announced 10 cases of Chinese naval ships trespassing since November 2004.
A survey ship intruding into territorial waters was first reported in November 2021 and the following year saw five such incidents.
Under the U.N. Convention on the Law of the Sea, passage through territorial waters is permitted if the foreign ships do not engage in military or economic activities, including seafloor surveys.
Defense Ministry officials said they were still determining if that applies to the latest incident.