PURPOSE: To simplify the construction process of a marine structure by a method in which the heavier weight of the main components are rapidly completed by a portal crane, while semi-complete marine structures obtained are moved to a dry dock or building birth, and then the high portion of the structure is built up by a tower type level luffing crane.

CONSTITUTION: A working platform 1 is built in a building dry dock 5 and supported by blocks 6 with supporting plates 4 provided below the working platform 1. An unit strut 2a is lifted up by a gate type crane running on the shoulder portion 5a of the dock 5 and orderly joined with the upper part of the supporting plate 4. After heavier weights of main components, e.g., an elevating base 1 and struts 2, are built up in the dock 5, water is charged into the dock 5 to raise them by the buoyance of the elevating base 1, and the main components are towed to a repairing dock 5' and set. The remaining portion 2' of the strut 2 is built up by operating a tower type level luffing crane 9 provided on the dock shoulder 5'a in a freely running manner. Thus, the jib 9a of the crane 9 can be raised to as high as 9'a, and therefore, the high portion of the structure can be easily built up.