It is important to prevent excessive vibration in the superstructure of a ship, in order to provide comfortable habitability to ship's crew and to keep normal operative condition for navigation instruments. Vibrations observed in ships has the following features : (1) The vibration amplitude largely scatters as long as the vibration is induced by the propeller, because propeller exciting force fluctuates due to the change of flow velocity into the propeller. (2) The vibration does not always have a single frequency but have several frequencies, since there are several exciting sources in a vessel such as main engine, propeller and auxiliary machines. Therefore, the vibration of ship's superstructure cannot be reduced by installing a simple anti-vibrating device, but more sophisticated instrument is required. To satisfy the above requirement, an active mass damper driven by electromagnetic force has been developed. An active mass damper can reduce effectively the vibration of fluctuating amplitude as well as the combined frequencies induced by several vibration sources. Firstly, vibration characteristics of this damper were confirmed by the exciter test using model frame structure. Then the capability of the damper was verified by the measurement of a VLCC and a large bulk carrier. Vibration amplitude of these vessels were confirmed to be reduced to about 1/2 to 1/5 of the original level by this active mass damper.