To improve the measuring accuracy of a current value irrespective of its level, for a battery whose charging and discharging current fluctuates widely, of a motor-driven vehicle.

An instructed value V (L) is read in, and this value V(L) is compared with a discriminating threshold value V(1) (S1). If V (L) ≥V (1), as a result of the comparison, a current sensor for a larger current is selected to measure the current based on the output value of the current sensor (S2). If V (L)<V (1), a current sensor for a small current is selected to measure the current based on the output value of the sensor (S3). This constitution enables the current to be measured highly accurately for charging and discharging currents of the battery that change widely, irrespective of the largeness of the level by securing the dynamic range for the current measurements. As a result, the residual capacity of the battery can be known.