To easily execute a work by providing a through-hole extended in one direction within a shell material, and penetrating the shell material in a prescribed length so that the through-hole of the shell material allows a river bottom or sea bottom to communicate with the water surface.

Four steel plates 11a, 11b, 12a, 12b are connected by welding to form a cofferdam box 10 having openings 13, 14 on both ends. When a cofferdam is constructed, the lower opening 14 of the cofferdam box 10 is placed on a river bottom 21 or a groove bottom. Further, a pressure is loaded from above the cofferdam box 10 to press fit a penetrating part having a prescribed length into the ground to close the water of the river by the upper sections of the steel plates 11a, 11b, 12a, 12b, the water within the cofferdam is removed to form a constructing space for constructing a structure. Thus, the setting work and removing work of the cofferdam can be easily executed.