Unconsolidated sand with the maximum thickness of 40 m was found in the D4 unit of the Plio-Pleistocene upper Dainenji Formation in southern Minamisoma City, Fukushima Prefecture, Japan. The thickness of this sand is unique because the D4 unit is mainly composed of mudstone with thin-sandstone. This sand is mainly composed of coarse to fine grains. Mudstone blocks are found at the bottom of this sand, and the sand is overlain by upper mudstone. The lower part of the sand shows slump structure. This sand shows fining-upward succession, which indicates that deposition had progressed during reducing period of current energy. The boundary between the sand and the underlying mudstone shows that the sand buries a channel extending from WSW to ENE with 4.5 km of length and 1 km of width. It was also presumed that the sand occurs above SF γ, a tephra bed of the D4 unit. The thickness, facies and stratigraphic position of the sand are similar to those of “medium-grain sandstone” at the site of Fukushima Daiichi Nuclear Power Station (FDNPS). Therefore, a channel buried by thick and unconsolidated sand is likely to exist even at the FDNPS site.