A semiconductor device includes first and second Fin FETs and a separation plug made of an insulating material and disposed between the first and second Fin FETs. The first Fin FET 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 in a second direction perpendicular to the first direction. The second Fin FET 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 in the second direction. When viewed from above, an end shape the separation plug has a concave curved shape, while an end of the first gate electrode abutting the separation plug has a convex curved shape.