PURPOSE: To always execute writing with an optimum voltage by executing the writing at a low writing voltage first, checking whether the writing is completed or not, raising the voltage when the writing is failed, executing the writing again, and repeating the writing data until the writing is completed.

CONSTITUTION: When the data are written from an external part to a register 1, a controller 5 controls a writing voltage control device 4 so as to output a minimum voltage to be generated, and the writing to a memory cell 2 is executed. Next, the controller 5 controls the memory cell 2 so as to execute a reading, and when a result in which a read resuLt and the contents of the register 1 are compared by a comparator 3 does not coincide, the controller 5 re-controls the writing voltage control device 4 so as to raise the voltage, and the writing to the memory cell 2 is executed. Thereafter, until the compared result coincides, namely, until the data are completely written, the writing is repeated while the writing voltage is successively raised. Thus, the writing with the approximately optimum can be always executed.