A data processing system including a data processing apparatus, includes a user data storing unit that stores user data of a user who uses a first service, the user data including first authentication data used for logging in the first service and user specifying data; a determining unit that determines, when the user who uses the first service sets second authentication data used for logging in a second service different from the first service, user specifying data usable as the second authentication data from the user specifying data included in the user data; and a management unit that sets the user specifying data determined to be usable as the second authentication data in the user data of the user who uses the first service as the second authentication data, and manages the user data as user data of the user who uses the first service and the second service.