A method for manufacturing a semiconductor device including steps of forming a gate oxide film and a gate electrode on a semiconductor substrate; implanting impurity ions of the same conductivity as the substrate in an oblique direction at a first tilt angle to the normal line of the substrate and with a first acceleration voltage and dose, while rotating the substrate about the normal line thereof; implanting impurities of the same conductivity as the substrate in the same manner except for using a tilt angle to the normal line which is greater and a dose which is smaller than that of the first tilt angle and dose; and forming source and drain regions by implanting impurity ions of the opposite conductivity to the substrate into the substrate, followed by performing a thermal treatment.