Japan is a collection of islands, spreading in a wide bow in front of the eastern or Pacific coast of China, on the western side of the Pacific Ocean. More than 3,000 islands form this country, the largest ones are Hokkaido, Honshu, Shikoku, and Kyushu. It is today one of the world's leading industrialized countries, with the world's second-largest economy behind the U.S.A.. Despite the generally overpopulated impression foreign visitors get, it is only the 18th most densely populated country in the world. About 73% of the country is mountainous, and unsuitable for agricultural, industrial, or residential use. This resulted in an extremely high population density in the habitable zones, mainly the coastal areas.
Japan is divided into 47 prefectures, which are commonly grouped into regions. Honshu, by far the largest and most populated island, is typically divided into five regions. The other major islands constitute one region each. We have listed only prefectures with caves, which are about a half.
In Japan approximately 3,000 caves are known. The longest cave is Akka-do in the Iwate Prefecture, which is not open to the public. Its known length is at the moment more than 24km  and rapidly increasing. Japan's second-longest cave is the Oyama Suikyodo cave with a total length of 10km. It is located on Okinoerabu Island, north of Okinawa.
Japan has more than 70 public caves, not all of them are developed show caves. Unfortunately there is no source for a complete list of show caves, at least not in English, so our list is not very complete. It is based on a list published by the Speleological Society of Japan, but it includes a lot of wild caves.
Two main types of caves can be found on Japan, karst caves and lava tubes. Caves are called do in Japanese, typically the suffix -do is appended to the cave name, e.g. Abukumado is Abukuma Cave. Karst caves are generally called shonyudo, which means dripstone cave.