An integrated circuit device includes a substrate, having a front surface and a rear surface opposite to each other, and a fin-type active region defined by a trench in the front surface, a device separation layer filling the trench, a source/drain region on the fin-type active region, a first conductive plug arranged on the source/drain region and electrically connected to the source/drain region, a power wiring line at least partially arranged on a lower surface of the substrate, a buried rail connected to the power wiring line through the device separation layer and decreasing in horizontal width toward the power wiring line, and a power via connecting the buried rail to the first conductive plug.