An actively controlled anti-rolling system has been developed with a view of reducing the rolling motion of a ship by the movement of the mass controlled by actuator. It is of hybrid type combining the pendulum-base passive type with active type driven by a relatively small electric motor. The hybrid type has merits of (a) providing almost the same damping performance as active type (depending solely on actuators for obtaining controlled movement of damper mass), while requiring only a much smaller control force for moving the damper mass, and (b) continuing to function solely on its passive mechanism in the event of power failure. This system consists of a sliding mass on the rail shaped in a circular arc, and the compact, passive pendulum mechanism is realized that does not require a suspension structure such as a simple pendulum or spring mechanism. The driving force to control the movement of the damper mass is imparted from the electric motor through reduction gearing connected to a gear and pinion mechanism. The LQ control theory has been adopted for controlling the damper mass. At-sea experiments were performed with a ship (weight approx. 190t) to verify damping effects of the hybrid anti-rolling system. The rollings were reduced to about 1/3 in beam seas under the condition that the ship was stationary. The performance was further compared between hybrid type and passive type. Hybrid type promised better damping performance by 15% than passive type. The hybrid type also attenuated rollings with a forward speed of ship to about 1/2 in following seas, in the case of which passive type presented difficulty of providing damping effects markedly. Good agreement was obtained between measurement and calculation.