A long-term observation system has been developed by the present writers since 1992. The system consists of several sub-systems such as an acousic reflection profiling system for spatial disribution survey of halocline(Model SC-3), an underwater acoustic reflection measurement systems for long-term observation of halocline behaviour(On-line mode Model SC-2 and Off-line mode CL-2), a thermometry system using optical fiber distrituted temprature sensor(Model DTS-80) and a temperature salinity measurement system using multiple CT sensors(Model MCTH-2). The observations was successfully carried out from Sept. 30 to Nov. 2 at the middle part of River Ohashigawa in addition to the ship-survey using Model SC-3.