Provided are an information processing device, a data synchronization program, a data synchronization method, a data synchronization system, and a terminal device that suppress an increase in processing load and communication load in a situation in which a plurality of user terminals participate and data of the plurality of user terminals are synchronized. A server device is connected to a plurality of clients that synchronize an object with each other via a network, and includes: data reception means configured to receive a difference from a synchronization target object from a client having ownership of the object; data update means configured to update first, second, third replicas, and the like obtained by replicating the synchronization target object by using the difference; and data transmission means configured to transmit the difference to a client other than the client having the ownership among the plurality of clients.