As a rewritable nonvolatile semiconductor memory device is known the flash memory, which stores charges in charge storage layers, such as the floating gates, etc. of the memory cell transistors to thereby store information. In the flash memory, information is written by injecting charges into the floating gates, etc., and information is erased by ejecting charges stored in the floating gates, etc. In the N-type memory cell transistors, when negative charges (electrons) are injected, their threshold voltage Vt becomes high, and when stored electrons are ejected, their threshold voltage Vt becomes low. Binary information is stored by the written state of the memory cell transistors, in which the threshold voltage Vt is high and the erased state thereof in which the threshold voltage Vt is low.
As flash memories are known NOR type, NAND type, etc. depending on their internal basic circuit structures. In the NOR flash memory, a plurality of memory cell transistors MT, which are present in the identical column are connected to a common bit line BL.