In order to manage adequately natural river environment and flood prevention project, discharge observation in natural rivers and construction of related database are very important. A rod float method is most prevailing for discharge measurement in natural rivers. However accuracy of this method is not good because the float could not always follow the mainstream and subjectivity of observer can't completely be removed. Here we report development of AUV (Autonomous-Underwater-Vehicle) designed to measure mean velocity in an open-channel flow. A PID control method could remain the position of the floating-robot stationary in main stream, and then mean velocity could be evaluated reasonably by a rotational speed of screw propeller.