A sensor detects a target by transmitting and receiving an electromagnetic wave, and is arranged in a vehicle compartment. A user's mobile terminal is notified via a cloud system when a vehicle is parked, and a suspicious object around the vehicle is detected by monitoring a periphery of the vehicle using the sensor. An obstacle interfering with a periphery monitoring and disposed in the vehicle compartment is detected using the sensor prior to the periphery monitoring. A detected obstacle is moved so as not to interfere with the periphery monitoring.