The Island of Kyushu
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.
North Korea
No. Not even at closest point from the continent and Japan somewhere in the south. Not even on Dok-do (Disputed Island) between the countries. According to both Wikipedia and Wikitravel, on very clear days the Japanese island of Tsushima can be seen from vantage points in Busan, South Korea, and the mountains of Korea can also be seen from Tsushima. I guess binoculars would probably help.
The four countries closest to Japan are China, Russia, North Korea, and South Korea.
No, South Korea is not a peninsula.
Russia. The distance between the island of Tsushima, Japan and the coast of Korea is an estimated 65 km (about 40 miles). The Soya Strait, which separates Sakhalin Island, Russia from Hokkaido, Japan is about 40 km wide (25 miles).
Kangaroo Island is closest to South Australia.
The country is bordered by the Democratic People's Republic of Korea (North Korea) to the north, the East Sea (Sea of Japan) to the east, the East China Sea to the south, and the Yellow Sea to the west; to the southeast it is separated from the Japanese island of Tsushima by the Korea Strait.
The Korea Strait separates Korea and Japan. It lies between the Japanese island of Kyushu and the South Korean mainland. It is an important waterway for maritime transportation between the two countries.
About 3,000 islands belong to South Korea.
The only country that borders South Korea is North Korea. South Korea is also very close to Japan and China over the water.
When South Korea's export growth stalled some South Korean firms suggested that south Korea primary export problem was the weakness in the Japanese yen.