PURPOSE: To exactly stop the leak of water by a method in which an injecting pipe is fixed to a convex lens-shaped cross section formed When coupling an annular coupling pipe fixed to both sides of a steel tubular sheet pile with the coupling pipe of the other steel tubular sheet pile and a grout is injected.

CONSTITUTION: Annular pipes 14 are fixed to both sides of a steel tubular sheet pile 10, and a notch 16 is partly formed over the longitudinal direction of the pipe 14 to obtain a coupler. A water stopping agent injection pipe 18 is fixed to the inside of a convex lens-shaped cross section formed when one coupling pipe 16 is coupled, having many injection holes 20 in its lower part. The sheet pile 10 is driven and a water stopping agent is injected into the convex lens-shaped cross section 22 of the joint of the sheet piles 10. Water- stopping work for the joint of the sheet piles 10 can thus be exactly attained.