The image processing apparatus (data processing apparatus) stores data in a storing unit (storing means), inputs and outputs the data to and from the storing unit via a storage control unit (input-output means) and processes the data outputted from the storing unit with a control unit (processing means). The storage control unit inputs and outputs image data to and from the storing unit by the DMA method through a path via a DMA control unit and inputs and outputs other data such as a control instruction to and from the storing unit by the PIO method through a path via a PIO control unit. Image data to be inputted and outputted to and from the storing unit by the DMA method is encrypted in an input operation and decrypted in an output operation by an encryption/decryption unit provided on the input-output path for DMA method.