1. A playback device that performs playback of a content recorded in an optical disc in conjunction with execution of an application program supplied by a server, the playback device comprising: a storage unit operable to acquire the application program from the server, and store the acquired application in a local storage; a first judgment unit operable to judge whether the application program is legitimate based on digital signature information supplied by the server; an acquisition unit operable, if the first judgment unit judges negatively, to acquire conjunction condition information from the optical disc, the conjunction condition information showing any one of (i) unconditional permission for the conjunction, (ii) conditional permission for the conjunction, and (iii) unconditional prohibition for the conjunction; a second judgment unit operable, if the acquired conjunction condition information shows the conditional permission, to judge whether the application program satisfies a permission condition for the conjunction; a restriction unit operable, if the conjunction condition information shows the conditional permission and the second judgment unit judges negatively, to restrict the playback of the content in conjunction with the execution of the application program; a detection unit operable to detect a region code from the optical disc; and an estimation unit operable to estimate a region code based on a source of the application program, wherein if a region code assigned to the playback device, the detected region code, and the estimated region code do not match each other, the second judgment unit judges negatively, wherein the restriction unit permits the playback of the content by merging the content with the application program, and wherein the restriction unit restricts the playback of the content by not merging the content with the application program.