In a method of manufacturing a semiconductor device, a first fin structure, a second fin structure and a third fin structure, which extend in a first direction, are formed over a substrate. A first gate structure is formed over the first to third fin structures. The first gate structure extends in a second direction crossing the first direction. The first fin structure and the second fin structure are arranged adjacent to each other, and widths of the first and second fin structures in the second direction are smaller than a width of the third fin structure in the second direction.