Preferential water flow is the process in which water moves along preferred pathways through soil profile, sometimes leading to rapid penetration of soluble contaminants into the underlying groundwater bodies. In Andisols, downward water movement occurs mainly due to horizontally uniform water flow as described by the one-dimensional vertical Darcy's equation. Therefore, preferential flow phenomena in Andisols have not been studied in detail especially under field conditions. The objective of this study was to elucidate the incidence of preferential water flow in the subsoil of an Andisol in relation to the soil water conditions and the rainfall amount and intensity under natural rainfall conditions. The study was conducted in a flat field located on Tsukuba upland, Ibaraki, Japan. The annual precipitation and average temperature during the monitoring period of 1997–2003 were in the ranges of 987–1,531 mm and 13.6–14.6 ˚C, respectively. The soil was an Andisol (Hydric Hapludand) consisting of surface soil (0–20 cm), ploughsole (20–30 cm), and subsoil (> 30 cm). The soil texture was light clay to heavy clay, abundant in allophane and other amorphous materials. A large number of tubular pores with diameters of 1–2 mm formed by decayed plant roots were visible in the subsoil. The air entry value of the subsoil was 0 to –8 cm. No cracks were observed in the soil profile. The water table usually fluctuates around a depth of 2 m. Sweet corn (Zea mays L.) and Chinese cabbage (Brassica pekinensis Rupr.) were cropped every year in summer and in autumn, respectively. Volumetric water contents from the surface to depths of 30 and 100 cm and that at a depth of 1 m were measured by time domain reflectometry using Tektronix 1502B cable tester. Pressure potentials at depths of 90 and 110 cm were measured by using tensiometers. Matrix flow across a depth of 1 m was calculated by applying Darcy's law. Unsaturated hydraulic conductivity of the soil was determined by the steady state method using an undisturbed soil core with a height of 4 cm and a diameter of 10 cm. Preferential flow across a depth of 1 m was determined by the water balance method during and shortly after a heavy rain event; in this estimation, evaporation, root water uptake, and net surface runoff were assumed to be relatively negligible. Matrix flow was the dominant process of drainage in the subsoil of this Andisol. It amounted to 167–627 mm per year and accounted for 73%–84% of the total annual drainage to below a depth of 1 m. Preferential flow was detected only 2–7 times per year; nevertheless, it accounted for 16%–27% of the total annual drainage. The amount of rainfall required for preferential flow to occur varied from 22 to 118 mm; this correlated negatively with the initial water storage in the 1-m layer, which ranged from 559 to 642 mm. Initially dry soil conditions enhanced rapid water absorption into the soil matrix, and little absorption occurred when the soil was initially wet. No preferential flow was detected when the initial water storage was lower than 550 mm. The intensity of rainfall had a relatively minor effect on the incidence of preferential flow. The maximum pressure potentials at depths of 90 and 110 cm in each preferential flow event were mostly in the range of –10 to –40 cm, which were lower than the air entry value; however, these values were not always sufficient for preferential flow to occur. On the other hand, when the average pressure potential at depths of 90 and 110 cm exceeded –8 cm, most of the drainage occurred due to preferential flow. These results suggest that the air entry value was the threshold for the incidence of preferential flow, and that preferential flow was produced in the upper soil layers where the pressure potentials would have been higher than the air entry value. The maximum pressure potentials in the subsoil during preferential flow events may correspond to the necessary condition for preferential flow to reach a depth greater than 1 m in the subsoil. We conclude that the incidence of preferential flow in the subsoil of this Andisol was mainly affected by the initial water storage and the amount of rainfall in each event, and that the preferential flow may become the dominant process of drainage under shallower water table or more humid climate conditions.