PURPOSE: To obtain a device for detecting a residual capacity of a secondary battery which can detect the residual capacity of the secondary battery accurately even if load current, etc., fluctuates.

CONSTITUTION: A title item is provided with an amplifier 9 which outputs a magnitude corresponding to a current value of a charge current or a discharge current which flows through a detection resistance, an oscillation circuit which oscillates at an oscillation frequency corresponding the above magnitude, a counter 15 which counts the oscillation frequency of this oscillation circuit, and comparator 7 which switches a count direction of the counter to an increasing direction and a decreasing direction when the charge current and the discharge current are detected, respectively.