A semiconductor device and a manufacturing method thereof which is suited for forming both a transistor for a memory cell and a transistor for a high voltage circuit part on one semiconductor substrate, and moreover, has little deterioration of an electric characteristic in the structure that a sidewall insulating film in a shared contact plug part is removed is provided. An active layer is formed by performing an additional impurity injection on a part where a sidewall insulating film is removed in a forming portion of a shared contact plug. An insulating film is laminated in a high voltage circuit part and a sidewall insulating film of wide width is formed. According to this, a forming width of a sidewall insulating film can be made small in a MOS transistor for a memory cell part, and a forming width of a sidewall insulating film can be made large in a MOS transistor for a high voltage circuit part. Thereupon, in the high voltage circuit part, a source/drain active layer can be formed in the position more distant from a gate electrode.