An actively controlled anti-rolling system has been developed with a view of reducing the rolling motion of ship. It is a new type of anti-rolling systems to replace the conventional systems, such as anti-rolling tank and fin stabilizer, and is of hybrid type combining the pendulum-base passive type with active type driven by relatively small electric motors. This system is composed of a moving weight oscillating on rail shaped in circular arc, carrying the driving motors and reduction gears, and the passive, compact mechanism is realized. The driving force to control the movement of the moving weight is imparted from the electric motors through reduction gears connected to a lack and pinion mechanism. The actual system of a moving weight of 100 tons has been applied to oceanographic research vessel “MIRAI” (8672 GT.) for practical use. Prior to service at open sea areas, the control system has been designed based on the identified parameters of ship motion characteristics obtained by results of forced rolling oscillation test using anti-rolling system in the calm sea. Control performance and dynamic behavior have been analyzed by recorded data of ship motion and displacement of moving weight at open sea areas. It has been revealed that the system operated following the control law and provided performance to be aimed in design.