A state judgment unit (21) of an on-vehicle terminal (20) judges whether the vehicle is in operation by using outputs of the operation state of a mobile terminal (10), a key insert switch (32), an ignition switch (33), and a courtesy switch (34). A theft detection unit (22) includes a diagnosis processing unit (22a) and a monitoring processing unit (22b). When the vehicle is in operation, the diagnosis processing unit (22a) executes diagnosis of a human body sensor (41), a vibration sensor (42), and a microphone (43). When the vehicle is not in operation state, the monitoring processing unit (22b) monitors presence/absence of theft action by using the outputs of the human body sensor (41), the vibration sensor (42), and the microphone (43).