A semiconductor device includes first and second active regions and a field insulating film contacting between the first and second active regions, and a gate electrode structure traversing the first and second active regions and the field insulating film, wherein the gate electrode structure includes a first portion positioned across the first active region and the field insulating film, a second portion positioned across the second active region and the field insulating film, and a third portion contacting the first and second portions. The gate electrode structure includes a gate electrode having an insertion film traversing the first and second active regions and the field insulating film second active region, and a filling film on the insertion film. A thickness of the gate electrode in the third portion is different from a thickness of the gate electrode in the first portion and the second portion.