To provide an installation method for a water cut-off box for installing the water cut-off box efficiently without using large-scale supporting facilities and machinery and materials needed for the installation by letting the water cut-off box used in the work for repairing a steel sheet pile structure float on the surface of the sea and bringing the box into an optional prearranged installation section such as a comparatively narrow water channel or the like.

When installing the water cut-off box A of front opening type in the steel sheet pile structure 51 to carry out the work for repairing the steel sheet pile structure 51, the water cut-off box A is made to float on the surface of the sea by buoyancy tanks 20, 30, 40 provided on its outer face and is pulled in front of the steel sheet pile structure 51 in the prearranged installation section. Attitude and depth of the water cut-off box A are adjusted by adjusting buoyancy of the buoyancy tanks 20, 30, 40 to let a front face of the water cut-off box A oppose to the steel sheet pile structure 51 and abut it on a surface of the steel sheet pile structure 51 for the installation.