In the case an update is generated for shared data housed in each terminal that composes a computer system, a terminal uniquely determined for every update based on a predetermined prescribed algorithm simultaneously transmits update information to the other terminals. In addition, information relating to member data within the group is also treated as shared data. Discrimination numbers are assigned to update information generated for shared data. During simultaneous multicast transmission, a discrimination number is transmitted together with update information, and that discrimination number is also managed as shared data in each terminal.