A diagnostic equipment coupled to a vehicle network makes an inquiry about internal information, e.g., yes or no whether there is a logging-available memory area, with respect to each of a plurality of ECUs, and only to any of the ECUs making a response of yes, e.g., having such a memory area, a notification is made when some defect is observed. The internal information is then gathered from the ECU, and in response to an inquiry about the internal information coming from the diagnostic equipment, the ECU forwards back information about yes or no for logging thereto. When the ECU is available for logging, log data that has been periodically accumulated is forwarded back as a response with respect to a retrieval request therefor from the diagnostic equipment.