To provide a wide effective range independent on regular or irregular waves and prevent the troublesome operation of anti-rolling tank control equipment (a valve or a damper).

The opening/closing of the damper and the valve as opening/ closing control equipment for an anti-rolling reservoir is controlled at two positions. When an average rolling cycle is located nearer the outside of an effective range than a 1/2 dead zone, a counter is operated (Step S7, S8), and when it exceeds the set frequency of the counter (Step S9), the damper or the valve is changed over and a control pattern is changed (Step S5) for improving anti-rolling efficiency or avoiding the ill effect of the anti-rolling reservoir.