A brushless motor composed of a cylindrical stator and a magnet rotor rotatably supported in the stator is driven by a battery. The stator includes a multi-phase armature winding to which current is supplied from the battery through a drive-control circuit. The drive-control circuit includes switching elements that are selectively turned on and off to generate a rotating magnetic field in the stator. The switch-timing of the switching elements is controlled according to a rotational position of the rotor and the battery voltage. When the battery voltage is normal, the switch-timing is set to an optimum point so that a maximum efficiency of the brushless motor is obtained. When the battery voltage becomes low, the switch-timing is advanced to secure a required motor speed. Thus, the required motor speed is always secured without using additional devices such as a voltage stabilizer or a pulse-width modulation controller.