To provide the residual capacity detection device of a capacitor for accurately detecting a residual capacity not only in a discharge state but also in a charge state.

At first, a voltage sensor 1-1 detects battery, a current sensor 1-2 detects battery current respectively, and their detected values are supplied to a battery control device 5. The battery control device 5 calculates a voltage change amount from a previously detected voltage value and a newly detected voltage value, and a current change amount from a previously detected current value and a newly detected current value. Further, the internal resistance of a battery 1 is calculated by dividing the calculated voltage change amount by the current change amount. Next, the state of the battery 1 from the value of the battery current is judged in the discharge state or the charge state and refers to an internal resistance/residual capacity characteristic corresponding to the judgement result. The residual capacity corresponding to the previously calculated internal resistance is detected.