Salt water intrusion into the Shimanto River was measured on June 16 and 17, and December 12 and 13, 2004 at spring tide, using a SC-3 acoustic profiling system which determines the spatial distribution of the halocline with the measurements of water quality （temperature, salinity and chlorophyle a）. This system is an improved 200 kHz precision echosounder, that records the distribution of the halocline and the thickness of the salt-water layer as profile records. At the Shimanto River saline intrusion has been known to reach up to 9.8 km from the river mouth, and is known to reach in less than 8 km in the present days owing to the deposition of sandy sediments along the downstream area. Our observation has clarified that the saline wedge usually remains at the deep of 5.6 km, and that it exceeds sometimes upstream up to 7.5 km during flood tide. The behavior of the salt wedge at the moderately mixed rivers are clearly visualized by the present study.