There are many cases of damage to structures in the ground that cause lateral flow, and various studies have been conducted. This study is an experimental investigation of the behavior of piles in soft ground where the upper and lower layers move relatively laterally due to the presence of a weak layer sandwiched between relatively firm ground. . One of the cases in which such conditions occur is the problem of liquefaction in the intermediate layer, which has been attracting attention since the Hyogo-ken Nanbu Earthquake. In this experiment, a model ground layer with a layer thickness of 80 cm was prepared (the upper 50 cm layer was a hard layer and the lower layer was a weak layer of 30 cm). At this time, we selected some ground strength and pile spacing of the weak layer and investigated how those parameters affect the behavior of the piles. As a result, the following conclusions were obtained.
　1) The difference in the strength of the intermediate weak layer has a great influence on the behavior of piles.
　2) When the strength of the intermediate weak layer is extremely low, such as when the intermediate layer liquefies, a model in which the piles are rotationally fixed at the boundary between the weak layer and the hard layer and are forcibly displaced horizontally. can reproduce the phenomenon, and can be calculated by applying the Minato Lab method.
　3) The smaller the distance between the pile centers, the greater the ratio of the resistance of the piles to the lateral flow.