In marine construction in Okinawa, underwater excavators with divers on board have been used for a long time because the seawater has high transparency, which makes it possible to recognize the situation by direct visual observation. However, with the current way, divers have to work near the underwater excavators. Thus, it takes large effort to ensure the safety of divers. Therefore, in order to contribute to the improvement of safety and productivity, we have studied the application of machine guidance system, which has been used in the land-based construction, to underwater excavators.
　In this study, in order to apply the machine guidance system to leveling works of mound, we set a target of ±32mm for the height measurement error of tips of excavators measured by underwater machine guidance system. To meet this target, we selected commercially available sensors that can be used in underwater construction, and conducted elemental tests to confirm the accuracy. In addition, underwater tests on mound shape recognition by profile sonar were conducted in our water tank to confirm the characteristics of the surrounding situation recognition.
　After that, our underwater machine guidance system was constructed by installing sensors on our underwater excavator. Then, the system was used to measure the terrain by contacting the tip of the equipment on land and in water. As a result, it was possible to measure the shape with a maximum error of +12mm on the concrete surface where the contact position of the tip on land was stable. In the underwater test on a model made of 150mm crushed stone, the measurement error was larger because the contact position of the cutting edge was unclear. However, it was confirmed that the system could measure with an average error of +31mm and a deviation of ±17mm.
　Additionally, we installed our system on the underwater excavator TKM200-9 and used it as an auxiliary tool in an actual marine construction site. After the experiment, we interviewed the divers and discussed the possibility of remote operation.

Key Words: port construction, underwater excavator, machine guidance, tank test, sea experiment