To prevent the theft of a vehicle intake system by providing an authentication device or an electronic input device between an opening/closing device and an engine key switch, and outputting a drive permission signal to operate the opening/closing device only when the inputted number coincides with the authentication number or electronic code set in advance when an engine is started.

This theft preventing device 1 is constituted of a switching valve 5 provided on the air passage 13 of the air cleaner of an intake duct 21, an opening/closing device 6 opening or closing the switching valve 5, and an authentication device 2 provided on a circuit between an engine key switch 3 and the opening/closing device 6. The authentication device 2 is constituted of a power circuit, ten keys, a CPU, a memory and an I/O port. An authentication number set in advance is stored in the memory via the ten keys, the number inputted from ten keys is compared with the authentication number by the CPU, and an output signal is sent to the I/O port when they coincide with each other. The I/O port is connected to the opening/closing device 6 side and the starter switch 3 side. The normal drive is suppressed, the travel of a vehicle is blocked, and a theft can be doubly prevented.