PURPOSE: To improve the integrating accuracy of a discharging amount integrating device for motor driving circuit without being affected by the capacitance of an electrolytic capacitor and an impedance by integrating the discharging amount of a battery based on a voltage value obtained by sampling the voltage across both ends of a shunt resistor asynchronously to PWM signals while the PWM signals are supplied.

CONSTITUTION: A controller 21 samples the voltage across both ends of a shunt resistor 55 and finds the electric current flowing through the resistor 55 from the sampled voltage. The controller 21 finds the electric power supplied to the resistor 55 from the average value of the sampled voltages after calculating the average value and finds the consumption of the electric power from a battery by integrating the electric power. Since the voltage across both ends of the resistor 55 is sampled asynchronously to PWM signals and the electric current flowing through the resistor 55 is found and integrated based on the sampled voltage in such a way, the estimation of the electric current flowing to a motor driving circuit is not required. In addition, since this process is not affected by the capacitance of an electrolytic capacitor 56 and an impedance, the integrating accuracy can be improved.