Salt water intrusion into the Aganogawa River was measured on November 19 and 20, 2004, and October 16 and 17, 2005 at spring tide, using mainly a SC-3 acoustic profiling system and water quality meter called TPM CLOROTEC. Velocity, direction and depth were also measured at two stations in the latter observation. SC-3 acoustic profiling system is an improved 200 kHz precision echosounder, that records the distribution of the halocline and the thickness of the saltwater layer as profile records. TPM CLOROTEC is a towing-type water quality meter, and real time data of depth, temperature, salinity, turbidity and chlorophyll-a can be obtained on the ship. These two instruments are very useful to recognize time-space ditiribution and changes of the halocline. At the Aganogawa River saline intrusion has been known to reach up to 14 km from the river mouth, and has been wrongly interpreted to the cause of the Niigata Minamata Disease. Our observation has successfully visualized the bebavior of salt water intrusion into the Aganogawa River