North Korea
The four countries closest to Japan are China, Russia, North Korea, and South Korea.
The 38th parallel
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38th parallel
Mokp'o, Hwasun, Gwangju, Sunch'on, Kwangyang, Samch'onp'o, Sinhyon, Chinhae, and Busan are all pretty close.
North and South Korea are due west of Japan, across the Sea of Japan. North Korea is at the same latitude as the northern half of Honshu, the large central island of Japan. South Korea is at the same latitude as southern Honshu (and Tokyo), but farther north than either or Shikoku or Kyushu. The southwestern section of the Sea of Japan that borders South Korea is also called the Tsushima Basin, and the straits separating the countries at their closest point are called the Korea Strait.
The only country that borders South Korea is North Korea. South Korea is also very close to Japan and China over the water.
The Southern Ocean
The second country closest to Japan is South Korea, located across the Korea Strait, a 200 km wide channel. The closest country is Russia, located about 10 kilometers between Soisho-to island and Hokkaido.
The 38th parallel.
The Island of Kyushu