To prevent an earth wall breaking accident even when a support material is removed by installing and fixing pipes for an outside and an inside expansible for waling and making a shape free to fix except for a square with one side open by arranging a bolt hole on a fixed part.

An accident is prevented even when earth wall breaking takes place by reinforcing walings 5 on both sides to hold sheet piles C driven on both opened and drilled sides perfectly by support materials on a retaining of opening and drilling construction and having a safety auxiliary material even after removing the support materials. In this case, a pipe 2 for an inside comes to be in an inserted state on the inside of a pipe 1 for an outside of the safety auxiliary material, and earth wall breaking is prevented in an interval of length of the pipe 1 for the outside. Consequently even when the earth wall breaking takes place after removing the support materials fixed on the walings 5, it is possible to check opening and drilling width in a constant interval.