In a diagnostic method for use in a diagnostic system capable of communicating with an information processing apparatus, information is obtained about print volume of an image forming apparatus for a certain period, the information being entered in the information processing apparatus by a user, a determination is made as to which of several image forming apparatus is to be recommended, based on the obtained information, and information is sent to the information processing apparatus via a communication line such that the recommendable image forming apparatus that has been determined is displayed in the information processing apparatus.