A data synchronous system synchronizes, between servers each having a shared memory, data which are stored on the respective shared memories. The system includes a data writer which writes data into the shared memory in one of the servers and then generates write state information on the write state of data written in the shared memory; and a data communicator which reads out the written data and positional information about a position on the shared memory of the written data on the basis of the write state information, and transfers the read data and positional information from the one server to another or some other servers.