An object of the invention is to provide a theft prevention system that appropriately protects a vehicle from being stolen. A communication ECU 3, an engine ECU 4, a car navigation system 5 and an air conditioner ECU 6, all installed on a vehicle 2, communicate each other to assure electrical connection, or `communication.` When an abnormality is detected in terms of communication, both of the communicated device and the device that conducts communication check operation stop to work. If any one of the above devices is removed from the vehicle 2 while the vehicle 2 is stolen, this scheme of operation prevents the onboard devices from working correctly as a total system. Therefore, the scheme of operation greatly decreases motivation of theft, resulting in protecting the vehicle 2 appropriately from theft.