In order to establish a physical model leading to stable (uniform) and unstable (non-uniform fingering) flows in unsaturated soils, two-dimensional tests on continuous rainfall infiltration were carried out by changing not only the initial water content (air dry, 0.5%, 1.0%) but also the rainfall intensity (15,30,180 mm/h). The physical mechanism of fingering flow was discussed on the basis of precise suction measurements inside and outside the fingers. The results show that wetting fronts developed in unsaturated sandy soils can be classified into three types ; 1) fingering flow, 2) wavy front, and 3) plane front. Two types of fingering flow can be further distinguished by the difference of the swelling velocity ; i. e., low-swell finger and high-swell finger flows. In each of the low-swell and high-swell fingers, a core and a swelling zone are developed while a finger is growing. Based on suction measurements inside them, it is found that the water condition in the finger core changes from wetting to drying processes while the water condition in the swelling zone remains in the wetting process. This hysteretic behavior in the core can be explained by the moisture distribution changing along a finger from tip to tail. Fingers swell when water moves from a finger core to surrounding dry sand. Such a swelling process is mainly controlled by the hydraulic conductivity for the wetting process at a finger tail.