PURPOSE: To save the ground resources as well as construction cost, shorten the period of construction, and stabilize the offshore site by a method in which a floating structure is extensionally set by connecting shafts in the seaside direction of a reinforcing foundation provided on the seaside end of the construction site on the land side to form a construction site on the offshore side.

CONSTITUTION: Reinforcing foundations 21 and 22 where a holding bracket 23 on the land side, having a plurality of brackets and consisting of a reinforced concrete structure 22 supported by many steel piles 21 or concrete, are provided on the seaside end of a construction site 3 on the land side. Many floats 13 are then set, and a connecting shaft 20 is inserted into a bracket 24a provided to the holding bracket 24 on the end of offshore site 1, or the floating structure with a mooring column 18, and the bracket on the land side for connection. The coupling force of the floating structure 14 can thus be raised, the drifting of the structure 14 can also be prevented, and the safety, extendability and connecting reliability of the structure 14 can be enhanced.