A semiconductor device includes first and second Fin FET transistors and a separation plug made of an insulating material and disposed between the first and second Fin FET transistors. The first Fin FET transistor includes a first fin structure extending in a first direction, a first gate dielectric formed over the first fin structure and a first gate electrode formed over the first gate dielectric and extending a second direction perpendicular to the first direction. The second Fin FET transistor includes a second fin structure, a second gate dielectric formed over the second fin structure and a second gate electrode formed over the first gate dielectric and extending the second direction. In across section along the second direction and across the first gate electrode, the second gate electrode and the separation plug, the separation plug has a tapered shape having atop size smaller than a bottom size.