According to one embodiment, a semiconductor device includes a first insulating layer on an underlying layer, a first trench formed in the first insulating layer, and a first graphene layer provided in the first trench. The first trench comprises a bottom surface on the underlying and two side surfaces joined to the bottom surface, formed into a U-shape. The first graphene layer has a stacked structure including a plurality of graphene sheets. The plurality of graphene sheets each include a depression in a central portion. Portions of the graphene sheets located in an edge of the first graphene layer are each extended upward, which is in a direction opposite to the bottom surface.