PURPOSE: To enhance the stability by providing a working deck with an opening for carrying in and out large-size apparatus, furnishing a lid on the opening, and locating the deck in a low position.

CONSTITUTION: A marine structure 1 is provided with a lower opening 2 which opens to the sea surface, and a working deck 3 is installed in such a way as enclosing the opening 2. The deck 3 is provided with an opening 6 for carrying in and out large-sized apparatus 5. On one side of the deck 3, a lid 7 enclosing the opening 6 is furnished in such a way as rotatable round a hinge 8, and a dolly for opening and shutting the lid 7 is installed on each side in longitudinal direction of the lid 7. When the apparatus 5 is sunk to the sea ground, the dolly is moved and the lid 7 is rotated downward to open the opening 6, and then the apparatus 5 is transported to the opening 6 and connected with the tip of a riser pipe.