To provide an offshore structure capable of effectively reducing, with a simple device, a rolling caused by ocean waves and wind.

This offshore structure comprises an upper structure 4 having an underwater structural body 5 projected from a bottom surface to the underside and a floating body 3 having an outer shell body 6 extending to an underwater and filled with water therein. The upper structure 4 is installed on the floating body 3 with the underwater structural body 5 inserted into the outer shell body 6, and supported by an attitude control device 7 operated by a signal from a controller. The attitude control device 7 comprises an actuator 16 for vertical control and an actuator 17 for horizontal control. A motion detecting sensor 19 installed on the floating body 3 is connected electrically to the controller 18.