Information of pore fluid in a debris flow is unclear, despite its contribution to the fluidity of the debris flow. To observe sediment concentrations of the pore fluids in debris flows, we established a novel monitoring system in Arimura River Catchment, Sakurajima Island, southern Japan. A culvert, 1.0m in width, 1.5m in height and 2.0m in length, was placed along the stream bank. An intake of 0.8×0.8m, which is covered by steel grating, was equipped on the side of the culvert so that only pore fluid of debris flows was led inside of the culvert. Measurements of dielectric constants, as well as pressure heads, were conducted within the culvert and used for the calculation of sediment concentrations of the fluid. The sediment concentrations of the pore fluid were successfully measured for natural debris flows. The preliminary observation revealed that the volumetric sediment concentration of pore fluid varied from approximately 22to69%. Successive occurrences of debris flow caused decreases in the sediment concentration of the pore fluids with each surge.