PURPOSE: To efficiently control the primary motion to be induced by heaving, pitching, rolling, surging, and swaying waves without fixing a floating platform to the bottom of the sea, by connecting a mass stabilizer having the sufficient added mass formed of water surrounding the stabilizer so that it may be suspended beneath the lower side of a floating platform.

CONSTITUTION: An added mass stabilizer 24 is suspended beneath a strut stabilizing floating platform 22 by many tendons 22, namely, ropes or stretching materials, at a sufficient distance. A stabilizer system maintains suspending tendons 26 in the pulling state in relation to all loading state by providing the added mass stabilizer of a water weight, and the sufficient water mass (that obtained by the weight of water to be moved when the sunk weight and the stabilizer mass are moved) performs the specified operation. That is, since the mass acts as a fixing member according to external force, the floating platform can be so designed as to cope with the heave, pitch, roll, surge and sway motions as the primary motion.