A semiconductor device including a semiconductor substrate; a plurality of memory cell transistors aligned in a predetermined direction on the semiconductor substrate, each memory cell transistor provided with a first gate electrode including a floating gate electrode comprising a polycrystalline silicon layer of a first thickness, a control gate electrode provided above the floating gate electrode, and an inter-gate insulating film between the floating and the control gate electrode; a pair of select gate transistors on the semiconductor substrate with a pair of second gate electrodes neighboring in alignment with the first gate electrode, each second gate electrode including a lower-layer gate electrode comprising the polycrystalline silicon layer of the first thickness, an upper-layer gate electrode provided above the lower-layer gate electrode; a polyplug of the first thickness situated between the second gate electrodes of the pair of select gate transistors; and a metal plug provided on the polyplug.