Internal erosion is one of the most common failure mechanisms in hydraulic structures. The internal erosion is generally initiated by horizontal seepage in earth-filled embankments such as levees. With a newly developed apparatus we carried out a set of internal erosion experiments in horizontal seepage. The experiments used gap-graded granular soil to prepare the specimens. The experimental results revealed that, in a horizontal column, the upper part of the specimen is preferentially eroded, sequentially forming a dominant seepage area. When the erosion progresses, the dominant seepage area gradually expands from the upper part to the lower part of the column. At the particle scale, the localized fine-particle movement can be observed in the erosion process when the hydraulic gradient is relatively small, e.g., less than 0.62. During the early stage of erosion, the particles are rapidly eroded. With the erosion time increasing, the particle loss slows down and even ceases if there is sufficient time.