For enhancing the high performance of a non-volatile semiconductor memory device having an MONOS type transistor, a non-volatile semiconductor memory device is provided with MONOS type transistors having improved performance in which the memory cell of an MONOS non-volatile memory comprises a control transistor and a memory transistor. A control gate of the control transistor comprises an n-type polycrystal silicon film and is formed over a gate insulative film comprising a silicon oxide film. A memory gate of the memory transistor comprises an n-type polycrystal silicon film and is disposed on one of the side walls of the control gate. The memory gate comprises a doped polycrystal silicon film with a sheet resistance lower than that of the control gate comprising a polycrystal silicon film formed by ion implantation of impurities to the undoped silicon film.