A data processing system includes a data processing apparatus and a server. The server stores a first and second application, the first application to be used by the data processing apparatus after being installed onto the data processing apparatus, the second application corresponding to the first application. The data processing apparatus includes: a display that displays an HTML-based operation screen of the second application on a Web browser, the operation screen being available from the server; and a main body that executes processing. The server includes a processor that offers an API when an instruction to execute processing is given via the operation screen displayed on the display, the API to be used by the second application such that the main body executes processing as instructed. The data processing apparatus further includes a processor that interprets the API and that makes the main body execute processing corresponding to the API.