In a method of manufacturing a semiconductor device, a fin structure, in which first semiconductor layers containing Ge and second semiconductor layers are alternately stacked, is formed over a bottom fin structure. A Ge concentration in the first semiconductor layers is increased. A sacrificial gate structure is formed over the fin structure. A source/drain epitaxial layer is formed over a source/drain region of the fin structure. The sacrificial gate structure is removed. The second semiconductor layers in a channel region are removed, thereby releasing the first semiconductor layers in which the Ge concentration is increased. A gate structure is formed around the first semiconductor layers in which the Ge concentration is increased.