To provide a failure diagnostic device for a vehicle capable of more efficiently outputting a diagnostic result of each electronic control unit.

An on-vehicle network system includes a plurality of electronic control units (ECUs) for executing self-failure diagnoses and a master ECU for integrally controlling the self-failure diagnostic results by all the ECUs. The master ECU stores dialogue information showing the self-failure diagnostic results acquired from other ECUs (slave ECUs) and dialogue information corresponding to the self-failure diagnosis by the master ECU. In this configuration, the master ECU includes external communication means performing communication with a vehicle analysis tool provided at the outside of the vehicle. When a transmission request is received from the vehicle analysis tool (S210; YES), dialogue information according to the type of the transmission request is transmitted (S275).