Black tooth country (黒歯国, Kokushikoku, Original Chinese: 黑齒國) is an ancient Chinese legendary country of black-toothed people thought to be far to the east.
Many researchers believe it to be a real country, since it is mentioned in the so-called Wajinden of the Records of the Three Kingdoms as being located far southeast of Wakoku.
In Korea, there is a theory that Heukchi Sangji is related.
Some researchers believe that the country may be related to Indonesia, and as a basis for their argument, they believe that in Indonesia, betel chewing (called kimmah in Japan), which is made from betel nut and lime, and it is said that they sometimes talk while emitting reddish-black juice, which is also indicated by the route and geography of the waterways described in the "Records of the Three Kingdoms".
In The Tale of Genji, Suetsumuhana it is said「Ohaguro The mountain and the sea are the land of black teeth in the East China Sea, where women have black dyed teeth.」 In "The Selected Writings", it is described as follows.