A monitoring system which can monitor a plurality of vehicles (1) includes vibration sensing units (15) each sensing a vibration of the associated vehicle (1), a sensed time specifying unit (20b) which specifies an instant at which the vibration of the vehicle (1) is sensed by the vibration sensing unit, and an abnormality determining unit (20a) which determines, when a vibration of one of the vehicles (1) is sensed by the associated vibration sensing unit (15), whether the vibration is caused by preparations for theft with respect to the vehicle (1) or the vibration is caused by environmental influences on the vehicle (1). The abnormality determining unit (20a) carries out the determination on ground of sensing results obtained by the vibration sensing units (15) of the respective vehicles (1) and a result of specification carried out by the sensed time specifying unit (20b). With this, a possibility of incorrectly identifying a vibration not being attributed to preparations for theft as a vibration attributed to preparations for theft.