To provide a vehicle diagnostic method capable of confirming the state of the vehicle.

The vehicle diagnostic method diagnoses the vehicle equipped with a sensor (S1) which is mounted to the vehicle and issues a detection signal, a first calculation means (E1) which receives the detection signal and transmits the output signal, an output means (30) issuing the output based on the input signal to be input, and a second calculation means (1) which receives the output signal and transmits the input signal. A virtual signal generating means (2) where a virtual signal generating means (2) having an output signal to be input and issuing the virtual output signal is disposed between the first calculation means (E1) and the second calculation means (1), and the confirmation of the operation of the second calculation means (1) and/or the output means (30) is performed based on the virtual output signal issued by the virtual signal generating means (2). Since the output means not operated in the state of diagnosing the vehicle is operated, the vehicle diagnosis method can diagnose the system mounted to the vehicle simply and safely.