Vertical infiltration process of rain-and irrigation-water in unsaturated soil is not quantitatively understood, in spite of its importance for predicting natural disasters and environmental pollutions. This can be attributable to the existing sampling techniques of unsaturated soil water, which are not necessarily appropriate considering the mechanism of soil water flow; the tension-free-lysimeter cannot keep the soil moisture condition at the water-sampling profile identical to that at the

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... ural soil profile, and the tension-lysimeter cannot measure infiltration rate. Moreover, the quality of sampled water by the tensionlysimeter reportedly depends on the applied vacuum pressure. This study proposes a new sampling method of unsaturated soil water which minimizes the disturbance of soil moisture condition at the sampling profile. The newly-developed equipment consists of a porous plate connected to a vacuum system and two tensiometers. Soil matric pressure heads just above the horizontally-installed porous plate and at the same depth in the natural soil profile, are measured at 3-second intervals. The vacuum system is automatically controlled so that the readings of the two matric pressure heads match each other. The equipment was tested by both of laboratory and field experiments. The results showed that the soil moisture condition at the water-sampling profile was kept almost identical to that at the natural soil profile, and the water sampling rate was about the same as the natural infiltration rate.