In a CMOS semiconductor device having a substrate, a gate insulating layer formed on the substrate, at least one first polysilicon gate formed over the substrate in at least one PMOS transistor region, and at least one second polysilicon gate formed over the substrate in at least one NMOS transistor region, a total amount of Ge in the first polysilicon gate is the same as that in the second polysilicon gate, a distribution of Ge concentration in the first and/or second polysilicon gate is different according to a distance from the gate insulating layer, and Ge concentration in a portion of the first polysilicon gate adjacent to the gate insulating layer is higher than that in the second polysilicon gate. The Ge concentration in the portion of the first polysilicon gate adjacent to the gate insulating layer is more than two times as high as that in the second polysilicon gate. For example, it is preferable that the Ge concentration in the portion of the first polysilicon gate adjacent to the gate insulating layer is more than 20%, and Ge concentration in a portion of the second polysilicon gate adjacent to the gate insulating layer is below 10%.