A well was installed on the bank of the Gonokawa River for the observation of groundwater. The well depth is 50 m and it penetrates alluvial sand and gravels. Long-term observation of the freshwater and saltwater boundary(F/S Boundary)was attempted, using a multi-sensor salinity/temperature measurement system based on the XCTD, a newly developed simplified conductivity measurement system(4 m cable with 40 sensors), and a pressure sensor for measuring the fluctuation of water level. Observation was carried out successfully for one year from September 2001 to August 2002, during which fluctuations of F/S boundary were recorded. This boundary responds to tidal movements with no time lag, and also changes in accordance with abrupt changes of river discharge with little or no time delay. Seasonal fluctuation of F/S boundary of more than 5 m has been detected for the first time by the present long term observation.