To detect data related to a place where data are changed and to transfer them to a slave server asynchronously.

Even though read and write access of file data is performed from an application 11 of a master server 2m to an OS 10, a file filter 14 is provided in such a case, an API 12 is also provided between the application and the OS, and data related to file writing places is extracted among access data by the file filter or the API. A changed file detecting means 18 fetches the data of the writing places, writes an address and area relation data in every file name of a memory file correspondence table, also notifies a communication processing part 20 of them and transfers them to a slave server 2s. Thereafter, the changed file detecting means or the communication processing part transfers file data.