PURPOSE: To reduce erroneous detection and to prevent not only the unnecessary consumption of a charging current but also the irregularity of detection voltage, by performing alarm display when a battery was consumed on the basis of the detection signal of a voltage detection part by a display element.

CONSTITUTION: During a time when the voltage of a battery B2 is high, the output terminal 4 of IC7 has High output and a current flows through the route of transistor Q1(15)→lamp L8→resistor R3(10)→condenser C1(13) but, because the resistance value of the resistor R3(10) is taken largely, the lamp L8 does not light. When the voltage of the battery begins to lower, the voltage inputted to the terminal 1 of IC7 becomes low. At this time, when the condenser C1(13) has reached a full charged state, a High signal is inputted through the terminal 6 of IC7 and the output of the terminal 4 changes from High to Low and a current flows in the order of lamp L8→terminal 4 to allow the lamp L8 to light. The condenser C1(13) discharges through the route of resistor R3(10)→output terminal 4 after was fully charged. During the discharge of the condenser C1(13), the output terminal 4 again comes to high output and, therefore, the lamp L8 is turned OFF.