This paper interpreted Shilla place name writing Naeul(奈乙), and based on this, discovered medieval language vocabulary, ‘Nari>Nae[川], Woomeul[井], Yeoheul[灘], Byeol[遷], Nara[津]’, etc. which preserve the ancient Korean language ‘*Eul[水]’. It was comprehended that Shilla place name writing Naeul expressed around ‘Nal’, and ‘Nal’ was comprehended to be originated from the composition of around ‘Na[壤]+l[水]’, which was comprehended to be the previous form of medieval language ‘Nari’. Based on this opinion, this research suggested a fact that all of the medieval language ‘Woomeul[井], Yeoheul[灘], Byeol[遷], Nara [津], Nari[川]’, etc. are the vocabulary whoc preserve the ancient Korean language ‘Eul[水]’. In other words, this research confirmed that ‘Woomeul[井]’ was from ‘Woom[音]+eul[泉]’, ‘Yeoheul[灘]’ was from ‘Yeo[石]+eul[泉]’, ‘Byeol[遷]’ was from ‘Byeo[石]+eul[泉]’, ‘Nara[津]’ was from ‘Na[水草]+eul [泉], ‘Nari[川]’ the previous form of the medieval language ‘Nae’ was from the composition around ‘Na[壤]+eul[泉]’. And confirmed that Woomeul[井] was ‘underground water’, and ‘Yeoheul [灘], Byeol[遷], Nara[津], Nari[川]’, etc. were ‘water on the ground’, in opposition, and divided accordingly. ‘water on the ground’, again, was confirmed to be divided into Nari[川], which has no characteristic and ‘Yeoheul[灘], Byeol[遷], Nara[津]’, etc. which have characteristic in opposition. In other words, it was confirmed to be divided that Yeoheul[灘] and Byeol[遷] were ‘Water with stone’, and Nara[津] was ‘Water with mud’ opposingly. It was assumed that Yeoheul[灘] was a vocabulary based on Koguryeo language and Byeol[遷] was Shilla language basis. There was only the meaning of ‘crossing place’ in medieval language, ‘Nara’, however it was confirmed that it also had the meaning of ‘Waterside with water plant(水草)’ in the period of the Three States, and that the original meaning of ‘Nari’, the previous form of medieval language Nae(川) was ‘water on the ground’.