Abstract: Disclosed herein is a device for tracking the position of a freshwater-saltwater interface of underground water in the coastal area. The device includes a buoyancy adjustment unit which comprises a pipe that has an internal space and is closed on a lower end thereof, a sealing cap which is removably coupled to an upper end of the buoyancy adjustment unit, a variable metal member which is attached to the sealing cap, a cordless measurement sensor which is provided under the lower end of the buoyancy adjustment unit, and a perforated pipe which is coupled to the lower end of the buoyancy adjustment unit and covers the cordless measurement sensor. Distilled water is supplied into the buoyancy adjustment unit, and a buoyancy of the buoyancy adjustment unit is adjusted depending on the amount of distilled water.