PURPOSE: To perform an erecting process of a columnar object by means of a single marine crane, by a method wherein one end of a columnar object, floating in a horizontal manner, is lifted in a condition that the other end is supported with buoyancy.

CONSTITUTION: A loading hang piece 23 is fastened to one end of a stud 21 on an assembly table 24 to assemble a columnar object formed in a perfectly water-tight structure. The stud 21 is lifted in an attitude that it has racks at both sides and hand pieces 23 at an uoper and a lower part and is positioned in a balancing condition between hangings performed by means of a factory crane and a marine crane to float it to the sea surface and tag it to a desired location by means of a tag boat. By means of a single marine crane 31, the loading hang piece 23 is lifted to bring it into an erecting condition in a condition that the other end of the stud 21 is supported with buoyancy, and the stud is loaded to a platform for the assembly.