The purpose of this study is to clarify the influence of entering fresh water from rivers into Tanabe bay area. The water temperature of fresh water is one of an important factor for ocean flow structure and three-dimensional structure of water temperature and salinity in coastal area. Fluctuation of river water temperature is repeated every day and seasonally, and it is influenced by elevation, atmospheric condition, confluence of river branch, groundwater, and water utilization of upstream area. In this study, we have conducted field measurement of river water temperature in neighboring rivers of Tanabe bay and will continue it for the next few years, in order to create estimated equations for river water temperature based on our measurement results. In this paper, we reported measurement method and initial progress of measurement results.