A method of manufacturing a semiconductor device includes the steps of forming a gate electrode of a transistor on an insulator layer on a surface of a semiconductor substrate, forming an isolation region by performing ion implantation of an impurity of a first conductivity type into the semiconductor substrate, forming a lightly doped drain region by performing, after forming a mask pattern including an opening portion narrower than a width of the gate electrode on an upper layer of the gate electrode of the transistor, ion implantation of an impurity of a second conductivity type near the surface of the semiconductor substrate with the mask pattern as a mask, and forming a source region and a drain region of the transistor by performing ion implantation of an impurity of the second conductivity type into the semiconductor substrate after forming the gate electrode of the transistor.