River discharge is one of important hydrological quantities for river design, river management, and disaster prevention, etc. A river discharge measurement system with high accuracy has been developed by combining hydroacoustic technology with numerical simulation of river flow. This new measurement system, ATENAS (advanced technology of numerical simulation of flow velocity distribution and hydroacoustics), was installed at the downstream region of the Tone River where river width is 500m or wider to verify its measurement performance. Mean flow velocity was successfully measured stably and continuously even along the long acoustic path of 590m by using ATENAS. The measurement accuracy of flow velocity and discharge was comparable to or higher than that of precise measurements by an acoustic velocity profiler.