To close in a short work execution period and maintain high cut-off.

A closing caisson comprises those divided container bodies 2 and 3 which are divided vertically into multiple bodies and of which buoyancy can be adjusted. Also one side of each of the divided container bodies 2 and 3 is hinged so that it can be opened and closed freely, and cushioning seal material 13 and supporting legs are installed continuously at the bottom of the cutting edge of each of the divided container bodies 2 and 3. Then, using multiple guide spacers 11 are installed inside the divided container bodies Using this closing caisson 1, each of the divided container bodies 2 and 3 is opened and closed so as to mount them on a structural body A for sinking. In addition, the cushioning seal material 13 is brought in contact elastically with a pedestal B for seating, and water-tight grout 18 is filled along the cutting edge part of the caisson 1 so as to cut off the cutting edge part of the caisson by the cooperation of the cushioning seal material 13 with the cut-off grout 18.