A semiconductor device can include an isolation region that defines a plurality of active regions. The plurality of active regions can include an upper surface having a short axis in a first direction and a long axis in a second direction. The plurality of active regions can be repeatedly disposed along the first direction and along the second direction, and can be spaced apart from each other. The isolation region can include a first insulating layer being in contact with side walls of a short axis pair of active regions which can be the closest active regions in the first direction among the plurality of active regions, and continuously extending along a first shortest distance between the short axis pair of active regions.