A vehicle management ECU comprises a diagnosis section for diagnosing electronic control units that are to be diagnosed according to a diagnosis scenario defined by a diagnosis application, and a diagnosis scenario determination section for determining whether a diagnosis scenario used for diagnosis by the diagnosis section was appropriate. If the diagnosis scenario determination section specifies that the diagnosis scenario used for the present diagnosis has not been appropriate, the diagnosis section carries out diagnosis according to a diagnosis scenario defined by a new diagnosis application different from the present diagnosis application.