PURPOSE: To perform the read/write checking without destructing a storage, by shunting temporarily the data stored in an RAM which holds the storage with use of a battery, etc. to a buffer memory to perform the read checking for each bit and then returning the data to the RAM. after the checking is over.

CONSTITUTION: A partial address A1 of the contents of an RAM (C) which holds the storage with use of a battery, etc. is shifted to a buffer memory D of a byte. Then the read/write checking is carried out. After the checking is over, the address A1 is sent back to the RAM. This procedure is carried out over the entire region of the RAM. As a result, the read/write checking is always possible without destructing the stored data to improve the maintenance properties.