The present disclosure provides a precise performance three-dimensional ultrasound scanner that is lightweight for an easy hold by suggesting a novel constitution of a swing mechanism wherein an arrangement of the drive motor perpendicular to the swing axis requires a smaller capacity motor and eliminating a backlash. The swing mechanism includes: an arm holder connected to a rotational axis of the motor; an arm connected to the arm holder so that the arm swings within a certain angular range; a link, attached to the arm, for moving in unison with the arm; a shaft, connected to the link, for transmitting a rotary power of the arm to the transducer unit. The swing mechanism can be operated using the smaller capacity motor and eliminates the need for a pulley, gear or belt eliminating backlashes to promote implementation of lightweight precision three-dimensional ultrasound scanners.