Hydrological observations at hillslopes revealed that chains of connected macropores often develop in the surface soil layer. These chains of macropores are referred to as soil pipes and the water flowing through them is called pipe flow. Numerous studies observed that the pipe flow greatly contributes to the rapid transfer of storm water. Thus, the importance of the pipe flow on hydrological processes in hillslopes has been pointed out by field observations. On the other hand, there are few studies which clarified processes of soil pipe developments. This study conducted a flume experiment to observe change in soil permeability affected by seepage forces. Mixture of silty clay and coarse sand was packed to a thickness of 30 cm in a flume, 90 cm long and 2 cm wide. The flume was inclined at 16 degree. Stainless steel wire-nettings as well as cotton cloths were attached between the soil-filled flume and lower and upper water tanks to support the soil. The water levels in the lower and upper tanks were fixed, and water was continuously supplied to the upper tank by using a rotary pump. Overflowing rate from the lower tank was automatically monitored. Two dimensional spatial distributions of pore water pressure in the soil were monitored using 32 tensiometers installed at 10 cm intervals. The experiment was continued for 3885 h. For 0 to 1080 h, the discharge rate increased gradually. Then, the discharge rate showed a rapid increase for 1080 to 1944 h, followed by a slight decline for 1944 to 3020 h. After 3020h, the discharge rate was almost stable. Tensiometer data indicated that the initial increase in the discharge rate was associated with increases in soil permeability in upstream regions. The following rapid discharge increment was caused by increases in soil permeability in downstream regions.