A semiconductor device includes an active region on a substrate; channel layers on the active region spaced apart from each other and including lower and upper channel layers; an intermediate insulating layer between an uppermost lower channel layer and a lowermost upper channel layer; a gate intersecting the active region and including a lower gate electrode surrounding the lower channel layers and an upper gate electrode surrounding the upper channel layers; an insulating pattern between the upper and lower gate electrodes on a side of the intermediate insulating layer; source/drain regions on at least one side of the gate, and including lower source/drain regions connected to the lower channel layers and upper source/drain regions connected to the upper channel layers; and a contact plug including a horizontal extension portion connected to the lower source/drain regions, and a vertical extension portion connected to the horizontal extension portion.