PURPOSE: To prevent a caisson from shifting or the like as well as to make its installation work once stoppable in a safety manner, by attaching a temporary cover body carrying the required weight detachably to the caisson in time of the worsening of a marine climate when the casting of stuffing sand, etc., takes place and the caisson is installed after it is mounted on the underwater foundation.

CONSTITUTION: In time of installation of a carisson 2, first of all, riprap is cast into a planned site for caisson installation in advance and an installing foundation 1 is created, then seawater is poured therein, and the caisson 2 is mounted on the specified position with a tugboat or a crane boat or the like. Next, casting of stuffing sand 4 takes place but at this time, if a marine climate is turns worse, some prearranged temporary cover bodies 3 having ample sharing weight in order to fix the caisson 2 on the foundation 1 are engaged with the foundation in a sate of almost blocking up the stuffing sand casting ports of each compartment 2a of the caisson 2 which is therefore made into a fixed state there, and then the foundation work is once suspended. And, after improvement of the marine climate, these temporary cover bodies 3 are removed, and casting the sand 4 is reopened.