Japan defined its territorial seas as extending 12 nautical miles seaward with the enactment of the Act on Territorial Sea and Contiguous Zone in 1977. However, there are still several sea areas where the breadth of territorial waters is 3 nautical miles, which are called as “specified sea areas.” In its appendix, the Act refers to the definition of an “international strait” in the United Nations Convention on the Law of the Sea (UNCLOS), when limiting the breadth of the territorial waters of Soya Strait (23 nautical miles wide), Tsugaru Strait (10 nautical miles wide), Tsushima Strait East Channel (25 nautical miles wide), Tsushima Strait West Channel (23 nautical miles wide), Osumi Strait (16 nautical miles wide) to 3 nautical miles, leaving high seas at the center of these straits. In doing so, it avoids the application in these straits of the “transit regime” adopted by the UNCLOS.
The UNCLOS has recognized “right of transit passage” for foreign vessels and aircraft (including foreign warships and military aircraft) to be able to exercise “freedom of navigation and overflight solely for the purpose of continuous and expeditious transit” of the international straits that connect international seas or exclusive economic zones (EEZs) with other high seas or EEZs. Although vessels navigating international straits are required to comply with certain rules established by the states bordering straits, freedom of navigation is guaranteed, and unlike territorial sea, navigation is not subject to the condition of innocent passage.
As an example regarding specified sea areas, between October 18 and 23, 2021, a total of 10 Chinese and Russian ships entered Tsugaru Strait from the Sea of Japan, navigated the high seas portion of the strait, headed south in the Pacific Ocean, navigated near the Izu Islands, navigated the high seas portion of Osumi Strait, and headed toward the East China Sea. A ship-based helicopter take-off and landing exercise was conducted in the sea areas around the Izu Islands and in the East China Sea. This was the first time that China and Russia had jointly conducted such a large-scale, long-running operation in the sea area around Japan.
If the Japanese government were to extend the breadth of its territorial seas of the specified sea areas from 3 nautical miles to 12 nautical miles, all the straits would be Japanese territorial sea, and foreign vessels would be able to exercise the rights of transit passage since they would be regarded as international straits under international law. Even if the specified sea areas are converted to international straits, this would not change the fact that foreign vessels could navigate them, but once they have become international straits, Japan as a state bordering international straits would be able to demand that foreign vessels conduct “continuous and rapid transit.”
Japan has many straits with a breadth of less than 24 nautical miles at their narrowest part other than the specified areas of sea. Examples include the Tokara Strait between Yakushima and Kuchinoshima (22 nautical miles wide), the strait between Akusekijima and Kodakarajima (18 nautical miles wide), the strait between Takarajima and Kaminonejima (21 nautical miles wide), and the strait between Tokunoshima and Okinoerabujima (18 nautical miles wide). All these straits consist entirely of Japanese territorial sea, and foreign vessels (including foreign warships) may exercise the right of innocent passage. The Japanese government has not explicitly stated whether international straits exist under international law in the Japanese territorial sea.
Since June 2016, Chinese warships have made multiple passages through Tokara Strait. The Chinese government claims that Tokara Strait is an “international strait” under international law. If this claim were to be accepted, it would mean that foreign submarines would be allowed to navigate in the Tokara Strait while submerged. The UNCLOS requires submarines to navigate on the surface and show their flag in territorial seas, but no such obligations are imposed while the submarines are in international straits. It is also permissible for foreign military aircraft to exercise freedom of overflight in international straits.
In November 2004, a Japan Maritime Self-Defense Force P3-C observed a submarine of unknown nationality navigating in the Japanese territorial sea around the Sakishima Islands in Okinawa Prefecture and moving northward while submerged. Since Japan requested it to navigate in the surface and would request it to leave Japanese territorial sea if it did not comply, the then-minister of state for defense ordered the Maritime Self-Defense Force to launch a maritime security operation. It was later identified as a Han-class nuclear submarine belonging to the Chinese People’s Liberation Army Navy.
Japan protested that the Chinese submarine had navigated Japanese territorial sea while submerged as a violation of international law, in response to which the Chinese government admitted that it had been a Han-class nuclear belonging to the PLA Navy, while explaining that the submarine had mistakenly entered Ishigaki Channel (18 nautical miles between Ishigaki Island and Tarama Island) for technical reasons in the course of regular training. This response by the Chinese government shows that the Chinese government regards Ishigaki Channel as the Japanese territorial sea and not as an international strait where foreign vessels have the right of transit passage.