To develop a new bedload monitoring method in pools of mountain streams, we attempted to establish a measurement system of sediment topography in a pool by utilizing TDR (Time Domain Reflectometry). Our measurement system consisted of cable tester, multiplexers and multiple sensor probes that were simply produced from a PCV pipe and two stainless rods. The measurement system was verified by a flume experiment, in which sediment was supplied to a flume and weighed in a container connected to the downstream end of the flume. Eight TDR probes were also installed in the container to measure sediment surface topography. Relative errors of sediment volume measured by TDR were around 10%, suggesting that our measurement system was useful even under running water conditions.