A state of charge of a battery (1) that drives a motor (6) is computed based on a discharge current measured by a current sensor (3) when the discharge current of the battery (1) does not exceed a rated discharge range of the battery (1). When the discharge current of the battery (1) exceeds the rated discharge range, the state of charge of the battery (1) is computed by estimating the discharge current of the battery (1) from a motor control signal, whereby current changes over a wide range can be found with good precision using the sensor (3), which has a narrow but high resolution measurable range, and the state of charge can be accurately detected.