A wiring structure includes a first insulation layer located on a substrate, and first and second plugs located on the substrate and extending through the first insulation layer. The first plug includes an upper peripheral portion that defines a recess and the second plug is adjacent to the first plug. A second insulation layer is located on the first insulation layer, the first plug and the second plug. A bit line structure is located on the second insulation layer and is electrically connected to the first plug. A protection spacer is located on the recess of the first plug and a sidewall of an opening in the second insulation layer. The opening exposes the recess of the first plug, the second plug and the sidewall of the bit line structure. A pad is located in the opening and contacts the second plug.