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 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 conditional permission for the conjunction, conditional permission for the conjunction, and 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; and a restriction unit operable, if the conjunction condition information shows the unconditional prohibition, or if the second judgment unit judges negatively, to restrict the playback of the content in conjunction with the execution of the application program.