An electrically rewritable flash memory device which has a memory cell array arranged in rows and columns of memory cells and which is divided into a plurality of memory blocks having different memory capacities. Each memory block having one or more rows of memory cells. A common voltage control circuit is provided for each of the memory blocks for applying a first potential to a common conductor for a memory block containing a memory cell selected with a selection voltage applied to its associated data line conductor for a writing operation and a second potential higher than the first potential to a common conductor for a memory block containing a memory cell unselected with the selection voltage applied to its associated data line conductor and containing no selected memory cell for a writing operation. A microcomputer having a CPU and the above-mentioned electrically rewritable flash memory formed in a single semiconductor chip includes an input terminal for receiving an operation mode signal for switching the microcomputer between a first operation mode in which the flash memory is rewritten under control of the CPU and a second operation mode in which the flash memory is rewritten under control of separate writing circuit externally connectable to the microcomputer.