PURPOSE: To decrease the exchanging number of an EEPROM by providing a block to hold the information of a constant byte number, an information holding area for writing propriety in each block, a RAM composed of the block of the same byte number as the EEPROM and a flag to show writing execution on the RAM.

CONSTITUTION: When the access of an EEPROM 2 is started, only a reloadable block is copied to a RAM 3 corresponding to reloading propriety information in an order from each block. When the holding information are worked, an access is executed to the RAM and the flag to correspond to the reloading information including block of a reloading control area is raised. Block information to have the flag are stored to the ROM 2. At such a time, the information are successively according to the same order. After the storage, the block information in the ROM 2 and RAM 3 are compared. When the information are noncoincident, an impossible flag is raised on the block of the reloading control area in the ROM 2 and in following operation, the blocks on all the RAMs are moved to the ROM 2. Then, when there is no block in the ROM 2, a warning is given. Thus, only the reloaded block is written and a writing number is decreased. Even in case that there is a troubled block, an information position is not changed when the block is developed on the RAM.