A secure processing device having a power saving mode, which is used for built-in apparatuses, calculates a hash value of secure data that needs to be saved when switching to the power saving mode, stores the calculated hash value in a protection storage unit whose data is not lost even in the power saving mode, encrypts the secure data and stores the encrypted data in an external memory when switching to the power saving mode. When switching back to the normal power mode, the secure processing device decrypts the encrypted data, calculates a hash value of the decrypted data and compares the hash value with the hash value stored in the protection storage unit. The decrypted data is restored to the protection storage unit when the hash values are identical, but discarded together with the encrypted data stored in the external memory when the hash values are not identical.