PROBLEM TO BE SOLVED: To enhance the efficiency of underwater tunnel construction work by connecting a plurality of unit box bodies in axial direction with connecting members coupled continuously, neighboring unit boxes are jointed through a water stop material thereby constructing sunk boxes, installing them on sea floor by connecting the sunk boxes together and reducing the section force acting to the sunk boxes. SOLUTION: A plurality of sunk boxes 1 are constructed by interposing a water expandable water stop material 4 having a high expandability to the jointing surfaces of a plurality of the unit box bodies 2, 2 and then jointing them together using a plurality of coupling cables 3, and a sunk box 1 is constructed. Unit box bodies 2a at the end portions of the sunk boxes 1, 1 butted together, bottom portions of both the notched parts are jointed with water stop plates 16, joint fittings 11 are jointed together, concrete 17 is placed in a recessed portion 9 consisting of the notched parts, a water stop grout material 14 is poured into the filled space 13 and is jointed to a rigid jointing state. Coupling between the sunk boxes 1 is in a state of rigid connection, but the unit box bodies 2 forming each sunk box 1 are coupled with a plurality of flexible coupling cables 3 and the water stop rubber 4, so that they are able to freely follow up differential settlement and crustal movement thereby decreasing the section force in the sunk box 1 in order to avoid the occurrence of a great section force.