A semiconductor device having a CMOS structure, wherein, in manufacturing a CMOS circuit, an impurity element which imparts p-type conductivity to the active layer of the p-channel type semiconductor device is added before forming the gate insulating film. Then, by applying thermal oxidation treatment to the active layer, the impurity element is subjected to redistribution, and the concentration of the impurity element in the principal surface of the active layer is minimized. The precise control of threshold voltage is enabled by the impurity element that is present in a trace quantity.