PURPOSE: To suppress the exhaustion of a battery on a portable device side and prolong the life of the battery by installing an electric conduction control means which cuts off the electric connection between the signal receiving part of the portable device and the battery on the basis of the information of the rhythm signal during the term from the time immediately after the completion of the transmission of an on-vehicle device to the time immediately before the next transmission.

CONSTITUTION: When the electric power switch of a portable device B is turned ON, and the monitoring signal 10 and the rhythm signal 22 which are supplied from an on-vehicle device A are detected, an electric conduction control means 6 cuts off the electric connection between a signal receiver 3 and a battery 5, during the term from the time immediately after the completion of the transmission of the on-vehicle device A to the time immediately before the next transmission, on the basis of the information of the rhythm signal 22 including the information related to the time (t) (about 10sec) up to the next transmission, and the exhaustion of the battery 5 is suppressed. If a person intrudes into an automobile and a vehicle detecting means 1 detects the theft action, the red lamp LED in the information part 4 of the portable device B is put ON by the monitoring signal 10 including the outputted anomaly information, and a buzzer operates, and the theft for the vehicle can be prevented beforehand.