PURPOSE: To easily measure the remaining power of a battery considering the ambient temperature by connecting a resistance element the resistance of which changes in accordance with the ambient temperature to an amplifier and changing the cumulation results of detected currents in accordance with the ambient temperature.

CONSTITUTION: An amplifier 3 obtains a signal proportional to the current flowing through a resistor 1 by amplifying a slight voltage drop across both ends of the resistor 1 for detecting current interposed between a battery 2 and negative pole and an adder 5a obtains a value representing the integrated value of the current flowing through the resistor 1 with respect to the time by adding the signal to the cumulated value of the current fed back from a register 5b. However, since the characteristics of the battery 2 generally depend on the temperature, the amplification degree of the amplifier 3 is changed in accordance with the ambient temperature by using a resistance temperature element 8 of a thermistor, etc., as the feedback resistor of the amplifier 3 and when the gain is raised in case the ambient temperature rises and lowered in case the temperature drops, the current flowing to the negative pole can be integrated with respect to time by taking into account the ambient temperature and the remaining power of the battery 2 can be accurately displayed 7.