This study develops an active vibration control unit for flexible booms for crop spraying. The unit consists of a fly-wheel, servo-motor, gyro sensor, and micro computer, and is termed an Active Wheel Damper (AWD). The AWD can be simply mounted on any flexible boom structure and enables effective damping of vibrations. The study further combines an adaptive disturbance cancellation controller and a sky-hook controller to obtain better vibration suppression performance, and names this controller a Sky-hook with Adaptive Disturbance Cancellation control (SWADC). The effectiveness of the developed vibration suppression unit is demonstrated by experiments of suppression of horizontal vibrations, which are in a direction parallel to the traveling direction of a spraying vehicle, of a flexible boom mounted on the vehicle pulled by a tractor. In the experiments, the AWD increases in damping for the motion of the deflection angle, which is the slope of deflection curve, of the flexible boom to suppress the translational flexible vibrations of the boom.