The control property of an onboard device is changeable by using an external control unit, and the data from the external control unit is authenticated. An onboard gateway unit is provided with a verification code generator for generating a security key, and an external control unit is provided with an application control unit for generating a security key. When the external control unit is connected, a main control logic unit of the gateway unit compares the two keys, and when the keys are verified, the control by the external control unit using an application program is enabled. The onboard device control unit can be easily connected to the external control unit. As the control of the device control unit by an external control signal is enabled only when verified, an unauthorized access can be prohibited, and the onboard deice can be controlled only by an authorized control signal.