According to one embodiment, a semiconductor device includes a substrate; a stacked body; a columnar portion; and a plate portion. The stacked body includes a plurality of electrode layers stacked with an insulator interposed. The electrode layers include first to third electrode layers. The first electrode layer is most proximal to the substrate. The second electrode layer is most distal to the substrate. The columnar portion and the plate portion are provided inside the stacked body. The plate portion extends along the stacking direction of the stacked body and along a first direction orthogonal to the stacking direction. The plate portion includes first to third portions. The third portion is provided between the first portion and the second portion. Widths of the first portion and the second portion along a second direction are narrower than a width of the third portion along the second direction.