PURPOSE: To save people aboard speedily and certainly in time of emergency, by installing a portal bridge in the rear side of a slide block being made rockable in a deck end part of a structure on the sea by an actuating cylinder, while slinging the end part of a sling wire stretched over a winch on an upper deck down from the portal bridge and engaging it to a lifeboat.

CONSTITUTION: A slide block 4 is rockable installed in a platform P of a marine structure in a state of its being projected to the platform P centering on a turning shaft 2. A lifeboat B is supported on the slide block 4 and its turning motion takes place with an actuating cylinder mechanism 10 being set up in position between the lifeboat and the platform. At the rear side of the slide block 4, a portal bridge 19 is installed by means of struts 17 and 18, while a sling wire 23, those one end is engaged to a winch 29 on the platform P, is slung down from the portal bridge 19 via a specified pulley block or the like and engaged with the lifeboat B. Thus, as an actuating mechanism is simplified as a whole, both free and gravitional drops of the lifeboat B and its recovery operaion are all effectively carried out with the actuating cylinder 10, the winch 29 and the specified lock mechanism.