[Problem] To provide an information processing device, a data synchronization program, a data synchronization method, a data synchronization system and a terminal device, which reduce an increase in processing load and/or communication load in a situation in which data of a plurality of user terminals are synchronized. [Solution] A server device 1 is connected, via a network, to a plurality of clients that participate in a virtual space. A synchronization object of the virtual space is synchronized between clients in a synchronization group that is composed of a client having the ownership of the object and a client not having the ownership of the object. The server device 1 comprises: a data reception means 104 that receives, from the client having the ownership of the object, a difference in the synchronization object; a data updating means 107 that updates, on the basis of the difference, replicas 120a, 120b, 120c, … which have been obtained by copying the synchronization object; and a data transmission means 105 that transmits the difference to the plurality of clients except for the client having the ownership.