A vehicular data communication system is disclosed. The vehicular data communication system includes an authentication device for authenticating an external tool connected to a bus, an authentication control device for determining whether an external tool is authenticated by the authentication device and for setting an authenticated state to permit a data communication between the external tool and an access target ECU on the bus upon determining that the external tool is authenticated by the authentication device, and an authentication maintain device for maintaining the authenticated state within a predetermined period after the authenticated state is set by the authentication control device.