To enhance reliability and performance of a semiconductor device that has a fully-depleted SOI transistor, while a width of an offset spacer formed on side walls of a gate electrode is configured to be larger than or equal to a thickness of a semiconductor layer and smaller than or equal to a thickness of a sum total of a thickness of the semiconductor layer and a thickness of an insulation film, an impurity is ion-implanted into the semiconductor layer that is not covered by the gate electrode and the offset spacer. Thus, an extension layer formed by ion implantation of an impurity is kept from entering into a channel from a position lower than the end part of the gate electrode.