At coastal area in Japan, marine construction works have been carried out in severe environmental conditions such as winds, waves and so on. Schedule of constructions are always affected by such sea conditions. Even if the wave height considerably small, operation rate of cargo handling on a floating crane may fall because the hang hook moves to every direction with ship motion and slinging works becomes difficult when the swell acts in the open sea. From this viewpoint, development of a system to decreasing vibration of the hang hook of a floating crane is expected eagerly.
　In this research, an experiment is conducted in a wave tank, we produced a scale model of a floating crane of rotary type that arm device to control vibration is equipped with. The amplitude of horizontal displacement of a hook is decreased by letting the arm device expand and contract. The effect of a system for decreasing vibration of hook is tested in variety of wave conditions. As a result, the amplitude of the hang hook of a floating crane is shown to be largely decreased by controlling arm device.