To appropriately prevent theft of a vehicle.

In this anti-theft system, each of a communication ECU 3, an engine ECU 4, a car navigation device 5, and an air conditioner ECU 6, mounted in the vehicle 2, mutually recognizes its connection, and, when it is detected that the connection between each of them and any other on-vehicle apparatus is not normal, stops operations of itself and the other on-vehicle apparatuses. Thus, even if the vehicle 2 is stolen and any of on-vehicle apparatuses is removed from the vehicle 2, these on-vehicle apparatuses are prevented from normally operating as an apparatus group. Therefore, theft intention on the theft side is sharply decreased, and the theft of the vehicle 2 is appropriately prevented.