A semiconductor device includes an etch stop layer, an insulating layer on the etch stop layer, and a contact structure passing through the etch stop layer and the insulating layer, the contact structure including a first conductive layer, a second conductive layer having a side surface and a lower surface facing the first conductive layer, a third conductive layer on an upper surface of the second conductive layer, and a natural oxide film between the first conductive layer and the second conductive layer and between the second conductive layer and the third conductive layer, the first to third conductive layers including metal or metal nitride, and the natural oxide film including metal oxide.