PURPOSE: To carry a large structure onto a barge without using large crane by conveying the structure while holding at water surface position as built on a surface plate on the barge sunk under water in advance and floating the barge.

CONSTITUTION: A surface plate 3 is carried through a float 2 onto the bottom sureface of a dock 1 as a foundation for installing at field, and a large structure P is assembled on the plate 3 with necessary pipes and devices. Then, water is introduced into the dock 1 to float the structure P together with the plate 3 via the float 2. A barge 4 for carrying a bearer 5 in advance on a deck is sunk in predetermined sea area to tow the structure P on the barge 4. Then, the ballast in the barge 4 is drained to float the barge 4 to thereby support the structure P on the plate 3 on the bearer 5 at the center so as to thus complete the carriage of the structure P onto the barge 4.