A server system includes a data storage section, HTTP server, back-end server, difference update instruction issuing sections, event management section, and real-time communication/transmission control section. The data storage section stores data. The HTTP server responds to an information transmission request sent from a client via the Internet, extracts requested information from the data storage section, and sends the information to the client. The back-end server sends an event associated with updating of the requested information. The difference update instruction issuing sections receive the event from the back-end server and issue difference update instructions to update the requested information. The event management section selects one of the difference update instruction issuing sections on the basis of the initial state associated with the requested information, and notifies the selected initial data instruction issuing section of the event. The real-time communication/transmission control section sends, to the client, a plurality of difference update instructions issued by the difference update instruction issuing section which is notified of the event. A client system and difference update system are also disclosed.