A MOS transistor has a first stress layer formed over a silicon substrate on a first side of a channel region defined by a gate electrode, and a second stress layer formed over the silicon substrate on a second side of the channel region, the first and second stress layers accumulating a tensile stress or a compressive stress depending on a conductivity type of the MOS transistor. The first stress layer has a first extending part rising upward from the silicon substrate near the channel region along a first sidewall of the gate electrode but separated from the first sidewall of the gate electrode, and the second stress layer has a second extending part rising upward from the silicon substrate near the channel region along a second sidewall of the gate electrode but separated from the second sidewall of the gate electrode.