Floating structure, floating wind-powered electricity generating device, and method for manufacturing floating structure

Provided are a floating structure, a floating wind-powered electricity generating device, and a method for manufacturing floating structure with which a wind turbine can be replaced and installed even if the size of the wind turbine is increased. A floating wind-powered electricity generating device 1 according to an embodiment of the present invention is provided with: a first wind turbine 2 which is driven in rotation by being subjected to wind, thereby generating electricity; and a floating structure 4 having buoyancy allowing installation of a second wind turbine 3 that may be installed in the future and that has a larger output than the output of the first wind turbine 2 actually installed. In this floating wind-powered electricity generating device 1, the floating structure 4 is designed and manufactured by calculating the required buoyancy on the basis of the total weight of the second wind turbine 3 having an output greater than that of the first wind turbine 2 that is actually to be installed, such t