PURPOSE: To make it possible to form long excavation holes for economical efficiency by digging down earth and sand in a wall surface protection pipe pressed into the ground with high pressure water and thereby turning the ground in a muddy state and forming excavation holes below and forming excavation holes one after another in the same manner.

CONSTITUTION: The tip of a first wall surface protection pipe 2 with a ring-shaped gasket 2a on the inner surface on the lower end is pressed in the round 1 substantially perpendicular to the ground. Then, high pressure water is sprayed from a jet nozzle at the tip of a lance 3 so as to dig out a ground surface. While muddy earth and sand are being discharged to the ground by means of a mud suction pipe 5, a greater part of the wall protection pipe 2 is pressed in the ground 1, thereby forming a first excavation hole (a). Then, a second wall surface protection pipe 6 mounted to a disk- shaped gasket 6a on the side surface on the lower end and a disk-shaped gasket on the outer surface on the upper end, is pressed in substantially perpendicular to the bottom of the first excavation hole (a), thereby forming a second excavation hole (b) below the first excavation hole (a) by the same operation. The disk-shaped gaskets 2a and 6b are engaged, which prevents the in-flow of underground water. The repetition of the same operation makes it possible to form an n+1 excavation hole in succession with the bottom of an n th excavation hole.