PURPOSE: To eliminate an error of the display of the remaining capacity of a battery due to temperature fluctuations without any adjustment, by detecting the liquid temperature of the battery by the use of a heat-sensitive element, and making a temperature compensation to the liquid temperature.

CONSTITUTION: A current detector S is provided to a terminal of the battery B to detect a current in the form of a voltage. An amplifier 1 consists of resistances R2WR5, capacitors C2 and C3, and thermistors TH1 and TH2, and amplifies the voltage detected by the current detector S. The thermistors TH1 and TH2 are coupled thermally by an element which has a negative temperature coefficient while decreasing in resistance according to a temperature rise, and attached in such a form that the temperature of the battery liquid is detected easily. Consequently, when the battery liquid temperature is low, the resistance is large and the amplification degree of an operational amplifier IC2 increases. Consequently, the amplification degree of the amplifier is large in winter to make a correction for increasing a voltage drop due to an increase in internal resistance, and a compensation for decreasing the voltage drop is made in summer automatically.