The present invention prevents a damage concentration caused by a data erase/write-in to a particular cell, so as to increase a service life of a flash memory. A CPU 17 determines whether a data which has been read out is a new one or a data having a reverse flag set (steps S1 and S2. If the reverse flag is not set, a reverse flag is set (step S3), so as to reverse the bit of the data when writing the data into a flash memory (steps S4 and S6). If a reverse flag is set, the reverse flag is reset (step S5) and the data is written into the flash memory (step S6).