Ribs 9'a to 9'd provided in a connector 50b on the terminal device side are coincident with grooves 8'a to 8'd provided in a connector 40b on the ECU 30b side, but are not coincident with grooves 8a to 8d provided in a connector 40a in position, size and shape. Even if the connector 40b of the ECU 30b without the immobilizer CPU 13 is attempted to be engaged the connector 50a which should originally be connected to the ECU 30a with the immobilizer CPU 13, it cannot be engaged or right electrical connections between input/output terminals in the ECU 30a and corresponding terminal devices cannot be achieved. Accordingly, the theft of the vehicle by replacing the ECU 30a with the ECU 30b is prevented.