To protect data from unexpected overwriting.

A control CPU 10 receives an overwriting request and reads a header content of storage data through a unit control CPU 33. An encrypted header content for downloading is prepared from the read header content, key information for encryption and decryption is outputted to the outside and an overwriting command is issued. The unit control CPU 33 reads the encrypted header content for downloading and requests the key information for encryption and decryption from the outside through the control CPU 10. The header content for downloading is decrypted by the key information to the request, and the decrypted header content for downloading is compared with the header content of storage data stored in a data storing part 32 to discriminate the existence/absence of a correlation. A writing means 33c receives correlation existence information, outputs a write enabling signal to the data storing part 32 and overwrites the storage data by write data between outputs of the write enabling signal.

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