A battery pack of a series combination of cell units is connected with a motor drive control circuit to drive a traction motor for a vehicle. A current sensor senses a discharge/charge current of the battery pack, and a voltage detecting circuit senses a voltage between two separate points in the series combination of the battery pack. A memory section stores a reference voltage drop quantity representing a decrease in voltage during a predetermined time interval between the two separate points. An offset detecting section compares an actual voltage drop quantity sensed by the voltage detecting circuit, with the reference voltage drop quantity, thereby detects a non-discharge/charge-current state of the current sensor, and reserves an output of the current sensor, as an offset quantity upon the detection. A correcting section corrects a sensed value of the discharge/charge current with the reserved offset quantity.