A data processing system is provided for quickly and appropriately handling alteration to shared data stored in main-memory areas of a plurality of machines connected via a network. The system has a transaction acceptor for accepting a request for executing the transaction, a transaction-execution manager for managing the execution of the transaction, a transaction executor for executing the transaction, a transcription processor for reflecting updated data sent from the base host on the replica data, a data monitor for monitoring whether data has been altered, and a register/deleter for registering/deleting positional information on at least a piece of data to be monitored for alteration to the data contents and an alteration-synchronizing transaction to be executed when the data has been altered, the alteration-synchronizing transaction being registered/deleted in relation to the data. On detection of alteration made to the data being monitored by the data monitor, the alteration-synchronizing transaction registered in relation to the altered data is read out and executed by the transaction executor.