To attain data flow control between different client programs based on a medium reproduction performance attribute for each program in the case of the data flow control between the client and the server interconnected by a network.

A client system 20 measures user activity information 22 denoting a notice degree of the user by a medium reproduction program 13 and media reproduction area information 23 or the like denoting media reproduction performance for each prescribed time. The measurement information is sent to a server system for each client program. The server revises data based on the measurement information such as reduction in transmission data as required and sends the resulting data to the client system 20. Thus, a band width of a limited communication medium and a computer resource of the server and client system 20 are effectively utilized without losing the visibility of the user.