PURPOSE: To lessen the rocking amount of a semi-submerged type marine structure as well as to improve the safety of the structure against inclination by forming a buoyant expanded portion at the connection between the upper deck and the column.

CONSTITUTION: An upper deck 3 is supported on columns 2 erected at four corners on the lower hull 1, and buoyant expanded portions 4 are attached to the connections between the columns 2 and the upper deck 3. When a structure is inclined to a state of water surface line B, the great inclination of the structure can be prevented because the buoyancy of the buoyant expanded portions 4 is worked and righting moment is increased.