A semiconductor device having active regions including a first active region and a second active region parallel to each other and respectively extending on a substrate in a first horizontal direction, a field region defining the active regions, a first insulating structure extending in the first horizontal direction on the field region, a gate structure extending, in a second horizontal direction, to intersect the active regions and the first insulating structure, source/drain regions disposed on at least one side of the gate structure, the source/drain regions including first source/drain regions on the first active region and second source/drain regions on the second active region, and a common contact plug on a first side of the gate structure and connected to the first and second source/drain regions opposing each other. The first insulating structure includes a first portion overlapping the gate structure in a vertical direction.