An illegal data use prevention system includes a registrar which issues
first authentication information for a game machine reproducing data and
second authentication information for a user-identifying IC card. The
data is identified by the first authentication information and the second
authentication information. The game machine contains the first
authentication information and, for example, executes a game program in
accordance with a result of determination as to whether the IC card
contains the second authentication information. Thus, illegal use of data
is effectively prevented.