Stream data processing system and processing method

The stream data processing system has an input server which accumulates input stream data to be migrated, instructs a migration source and a migration destination to perform data migration, transfers accumulated data to the migration destination in response to an accumulated data transfer instruction, then stops transferring data to the migration source, and transfers subsequent data to the migration destination subsequently to the transfer of the accumulated data. The migration source migrates both a query and an intermediate value that has been produced as a result of completion of execution of the query on input data. A migration destination server reads the accumulated data and the subsequent data that are transferred from the input server, skips reading the portions of these transferred data corresponding to and preceding the migrated intermediate value, and discards the output that is transferred from the migration source and precedes the query execution results.