PURPOSE: To increase the using efficiency of a write area by comparing write data with read data whenever writing is executed to a prescribed bank, judging that the life of the bank is expired at the time of discordance, and using the next tank.

CONSTITUTION: First, the data are written by using a first bank out of the plural banks formed in a nonvolatile memory (EEPROM) 9, whenever the writing is executed, the data in an address are read, they are compared with the write data, when they are discordant, the first bank is judged to be expired, and the wall written data are shifted to the second bank, the data writing is executed by using the second bank from the next time, and thereafter the same processing as above is repeated. Thus, since the respective plural banks can be used until their lives are expired, the EEPROM 9 having the restriction of the number of times of rewriting can be efficiently used for a long time.