In a method of manufacturing a semiconductor device, a fin structure protruding from an isolation insulating layer disposed over a substrate is formed, a sacrificial gate dielectric layer is formed over the fin structure, a polysilicon layer is formed over the sacrificial gate dielectric layer, a mask pattern is formed over the polysilicon layer, and the polysilicon layer is patterned into a sacrificial gate electrode using the mask pattern as an etching mask. The sacrificial gate electrode has a narrow portion above a level of a top of the fin structure such that a width of the sacrificial gate electrode decreases, takes a local minimum, and then increases from the top of the fin structure.