We observed continuously behaviour of bottom water near the pipe exchanging with seawater in Honjou area, Lake Nakaumi, to explicate the effect of the pipe. We analyzed salinity, water temperature, flow velocity, direction and halocline behaviour by the systems which were developed for the long-term observation of halocline behaviour in estuaries. Although the difference between incoming seawater and the indigenous water in Honjou area was small, the result suggested that (1) the bottom water mass was influenced by seawater inflow near the pipe and (2) more than 50m away from the pipe, the bottom water was not affected by seawater incoming. We also found the other phenomenon, i. e. the high temperature water mass moving. The equipments that we used were very useful to observe the water behaviour in the field where the variation of water property was small.