The Tsukuba Upland, about 390 km2 in area, is one of the diluvial uplands commonly found in the Kanto Plain (Fig. 1). The upland is composed of the Younger Kanto Loam (about 3m thick at most), the Joso Formation, and the Kioroshi Formation in descending order. In the upper part of the Joso Formation is a less-permeable clay/silt layer called the Joso Clay (Fig. 2). The present study aims at clarifying the physical properties of the Joso Clay, which are important from the hydrological point of view. The Joso Clay was sampled at 19 points on the Tsukuba Upland for determination of its physical properties (_??_ in Fig. 1). The Younger Kanto Loam and the sands, situated above and below the Joso Clay, respectively, were also sampled for a comparison of physical properties. In addition, some data on physical properties of the Joso Clay were obtained from the literature (0 in Fig. 1). Saturated hydraulic conductivities of the Joso Clay proved to be extremely low, ranging from 8.29×10-6 to 5.30×10-9cm/sec with an average value of 1.39×10-6cm/sec (Table 1). The Joso Clay presented a remarkable contrast in saturated hydraulic conductivity to both the overlying Younger Kanto Loam and the underlying sands, showing differences of two to three and three to five orders of magnitude, respectively (Fig. 3). The porosity of the Joso Clay was between 54% and 77%. The Joso Clay showed a high degree of variability in thickness, from 15 to 400cm (Fig. 4). No macropores or non-capillary pores with the pore diameters larger than 3.8×10-2mm were found in the Joso Clay (Fig. 5), indicating that almost all the water in the Joso Clay moves very sluggishly. Based on the results of this study, vertical subsurface water movement in surface parts of the Tsukuba Upland is assumed to be substantially restricted due to the presence of the Joso Clay.