PURPOSE: To improve the water-tightness of an underwater structure as well as lessen the area of the ground to be graded by a method in which one edge of a bottom cover is movably attached to the inside of a bottomless box, the box is settled under the condition that no bottom is present, the bottom cover is fallen to form the bottom, and a filling material is charged into the box.

CONSTITUTION: A partition plate 11 is provided in a box 1 to form compartments 12 whose upper and lower ends are opened independently. On end of a movable bottom 2 is pivotally attached to the lower end of the partition plate 11, and a rope 3 is attached to the other end of the bottom 2. In the construction work, the rope 3 is pulled up, the box 1 is delivered while keeping the bottom 2 almost vertically to a construction site, and the box 1 is settled. Under the condition, coarse aggregate 4 is charged onto the bottom. The rope 3 is then loosened, the bottom 2 is fallen to cover the bottom of the compartments 12, and a filling material (not illustrated), e.g., sand, etc., is charged onto the bottom to complete the construction work. The water-tightness of the underwater structure can thus be improved, and the area of the ground to be graded in the setting point can be lessened.