[en] The flow rate of underground-water in deep geologic environment is a key parameter to evaluate the performance of the repository for high-level radioactive waste disposal. The preliminary estimation however suggested the flow rate is much lower than the detection limit using conventional measurement techniques. This situation may underestimate the repository function because of the lack of the actual measured data. Thus the authors developed a new technique for the three-dimensional flow measurement by using the ultrasonic echo reflected from solid particle tracers suspended in groundwater. The moving track is traced by the matrix arrayed sensor and this technique enables us to measure the wide range of flow rates of 10^-10-10^-5m/s. The preliminary experiments hence suggested the tracer must be stably suspended for a period not shorter than two days. To realize this requirement, the disturbance of sedimentation due to density difference between the solid particle and groundwater and Brownian motion of the particle must be estimated and eliminated. We designed and fabricated the solid particle tracer having a core covered with a shell of different density and the resulting density is easily adjustable. The stability of the tracer was examined by in-situ experiment carried out in 500m depth bore hole and successfully measured the flow rates of 10^-10-10^-5m/s. (author)