To provide a cycle setting method for a variable cycle antirolling water tank device allowing the proper setting of an effective cycle range of an antirolling water tank for effective hull rolling control of a ship with the variable cycle antirolling water tank device mounted therein.

A hull rolling response curve P1 during non-operation of the antirolling water tank in regular waves, a hull rolling response curve P2 in an open-damper condition during operation of the antirolling water tank in regular waves and a hull rolling response curve P3 in a closed-damper condition during operation of the antirolling water tank in regular waves are produced so that the effective cycle range F (intersection points T1-T3) can be set, which consists of a cycle range between the intersection point T1 of the response curves P1, P2 and the intersection point T2 of the response curves P2, P3, as an effective cycle range F1 in the open-damper condition, and a cycle range between the intersection point T2 and the intersection point T3 of the response curves P1, P3, as a effective cycle range F2 in the closed-damper condition.