River water expansion (RWE) into a semi-enclosed coastal bay is one of the important elements for the environmental situation of the bay. In order to know RWE in the inner Kagoshima Bay, 16 images of the Landsat-5/TM band 6 are examined. These images are investigated in relation the tidal situation, the water amount of Amori river, the wind direction and the wind speed. Numerical simulation of tidal residual flow is also performed. The main results are as follows ; i) The river water normally assimilates to sea water in about 4 to 6.5 km off from the mouth of the river. ii) The area of RWE is affected by the wind speed during 17 hours before, especially 9 to 15 hours. iii) The direction of RWE is mainly westwards following the constant sea surface flow. iv) At the western region of the inner bay, the front of river water tends to appear in a form parallel to the north-west coast. v) When the amount of river water is large, it is difficult to distinguish the river water from the surrounding sea water. vi) Numerical simulation qualitatively explains the characteristics of RWE.