A system for monitoring coastal underground water is disclosed. The system includes a freshwater/saltwater interface position tracking device that is inserted into an observation well used for observing coastal underground water and can move on a freshwater/saltwater interface according to a change of buoyancy; and a first measuring sensor that is inserted into the observation well so as to be placed at a position above the freshwater/saltwater interface position tracking device and has a distance measuring unit used for measuring a first distance between the freshwater/saltwater interface position tracking device and the first measuring sensor, wherein the first measuring sensor has a first signal output unit used for outputting a signal indicative of the measured first distance.