PURPOSE: To prevent leaving personal belongings or theft thereof by issuing an alarm when signal communication is interrupted for a certain period of time between a slave unit mounted on each piece of the belongings and a master unit mounted on the possessor and when an acceleration is applied to the slave unit.

CONSTITUTION: A slave unit 2 to be mounted on each piece of the personal belongings and a master unit 1 to be mounted on the possessor for the belongings are equipped with respective signal transmitting part 13, 4, signal receiving parts 14, 5, alarm devices 17, 8, and control parts. When the leaving preventive mode is on, a normal state signal is issued at certain intervals from the master unit 1, and the slave unit 2 holds its alarm device 17 in the off state as long as the unit 2 is receiving such signals, When no signals from the master unit 1 are issued for a certain period of time, the alarm device 17 of the slave unit 2 is turned on, and alarm is issued. When the theft preventive mode is in effect, on the other hand, a sensor 19 of the slave unit 2 is turned on, and eventual application of an acceleration to the belongings and sensing of the same should cause actuation of the alarm device 17. It may be acceptable also that an alarm signal is passed to the master unit 1 to cause its issuing the alarm in case the slave unit 2 issues an alarm.