There is provided a non-volatile memory device having a multi-bit cell structure. In the non-volatile memory device, a memory cell array includes a plurality of cells of a first conductivity type which has different threshold voltages and are arranged in a matrix on a semiconductor substrate. A bulk region of a second conductivity type opposite to the first conductivity underlies the memory cell array and receives a predetermined back bias voltage when a cell is driven. The threshold voltage difference between states can be sufficiently widened because a state having a high bulk concentration is highly susceptible to a body effect. Therefore, reduction of masks leads to process simplicity, reduced turnaround time, and improved process margin.