To simplify a constitution used to prevent a trouble that data is disguised due to the interference of an answer from every on-vehicle electronic control device and to reduce an increase in costs to a minimum.

In this vehicle diagnostic system, an engine control device 10 and an AT control device 20 are connected by a communication permission flagline 4. By the engine control device 10, the finish of the transmission of failure information is notified to the AT control device 20 via the communication permission flag line 4. The AT control device 10 transmits failure information only when it receives its notice. As a result, while many communication protocols and a complicated arbitration program are not built in the system, the respective on-vehicle electronic control devices 10, 20 can send answers sequentially with reference to a request from an OBD scan tool 30.