To provide an antitheft security system of a vehicle, improve robustness against a theft of the vehicle through fraudulent access, and prevent the theft of the vehicle through fraudulent information acquisition.

A drive control unit (9) transmits a code to a control unit selected by a unit selector (10), produces a prediction value by predicting a response value to be returned from the control unit selected by the unit selector (10), and permits start of an internal combustion engine (1) if the prediction value squares with an actually returned response value. Plural control units (6, 7, and 8) include: at least two response algorithms (6A, 7B, and 8C) according to which the code sent from the drive control unit (9) is converted in order to obtain the response value. The drive control unit (9) includes: prediction algorithms (9A, 9B, and 9C) associated with the response algorithms so as to convert the code to be transmitted to the control unit selected by the unit selector (10) and obtain the prediction value.