To make it possible to accurately measure the residual capacity of a battery, by starting to judge the residual capacity of a battery after measuring the time elapsed from the start of motor drive after measuring time for a predetermined duration by a battery residual capacity judging section.

After measuring for a predetermined time by time measuring means 3E of a battery residual capacity judging section 3 based on a voltage value of a battery 12, the drive of a motor 11 is started and the time elapsed is measured, and the judgment of the battery residual capacity is started by judging start control means 3F. That is, the voltage of the battery 12 is not measured because the voltage of the battery 12 is unstable when the motor 11 has not be driven and until a predetermined time has passed after the start of the drive of motor 11, and the residual capacity of the battery 12 is measured based on the voltage of stabilized battery 12 after the elapse of a predetermined time from the start of drive of the motor 12.