A relay server computer adds control code to background contents to generate web data for display engagement, and sends the generated web data to every client computer. The control code becomes a controller by executed on the client. When an object such as image is pasted on the background contents displayed in an engaged fashion, the controller detects the event occurred by pasting, and sends the information indicating the event to the other clients via relay server. Consequently, the screen with the object being pasted is displayed in an engaged fashion among all clients. The object can be moved, modified, or deleted.