PURPOSE: To evade a data breakdown due to a malfunction by writing a protection objective address and protective information to an external storing device, and generating updated data based on the protective information when a write address corresponds to the protection objective address.

CONSTITUTION: When a command issued from a CPU 11 is the protective information setting command, the protection objective address and mask information in the address are continuously read from the CPU 11, and the information is written to areas 31 and 32 on external storages 13 to 15. When it is a write command, whether or not the write address is the protection objective address is judged, when they correspond, the mask information is read from the area 32, the data to be written are generated based on the mask information, the original data of the external storage, and the write data obtained form the CPU 11, and the data are written to the area 33. Thus the data breakdown due to the unexpected malfunction of an external storing device access program can be evaded.