A semiconductor device includes merged contact plugs. A multi-fin active having N sub-fins is formed in a substrate. A contact plug is formed on the impurity areas. N is an integer between about eight (8) and about one thousand (1000). The N sub-fins include a first sub-fin formed in the outermost portion of the multi-fin active and a second sub-fin formed near the first sub-fin. A straight line perpendicular to a surface of the substrate and passes through a virtual bottom edge of the contact plug is disposed between the first sub-fin and the second sub-fin, or through the second sub-fin. The virtual bottom edge of the contact plug is defined at a cross point of a correlation line extending on a side surface of the contact plug and a horizontal line in contact with a lowermost end of the contact plug and parallel to the surface of the substrate.