To provide a data processing apparatus which can protect data in a medium even if the user leaves the spot with the medium kept connected and forgotten at user's logout and then the third party carries away the medium, and moreover which can easily restore the data when a true owner of the data logs in a system again.

If a removable medium is connected at user's logout, the data within the removable medium is retreated to a storage region exclusive for the user, so that a state with no data in the medium is formed. Moreover, the logout is established. When the owner of the data logs in again, the retreated data shows its presence, and the data retreated to the present medium is rewritten (moved again) to restore the data into the medium.