To make the residual capacity of a battery beforehand detectable by making an alarming means output alarm at the time of discriminating that battery residual capacity falls below a preliminarily set value, based on a signal from a battery residual capacity detecting means when a battery residual capac ity detection command signal is inputted through an external signal inputting means.

A signal received through an antenna 15 is inputted to a demodulator circuit 21, demodulated and inputted to a CPU 9. A LED driving circuit 27 drives a LED 31 and also a buzzer driving circuit 29 drives a buzzer 11. And, the total consumption current value of a battery is calculated by adding consumption current value of the battery while alarm is issued to consumption current value of the battery while alarm is stopped. Next, battery residual quantity is calculated by subtracting the total consumption current value from the battery capacity of the battery and alarm is outputted appropriately while comparing the battery capacity with threshold.