To provide a storage device having a data concealment mechanism without imposing special processing to an arithmetic unit.

A storage device is characterized by having; a nonvolatile memory which stores data; a first volatile memory; a means which decrypts and converts the above data into decrypted data and transfers this decrypted data or the above data to the above first volatile memory; and an arithmetic unit which contains the above first volatile memory inside and processes the transmitted data. The storage device is characterized in that the above-means performs decryption of the encrypted data based on a direction from an external host and does not perform decoding processing to not encrypted data.