To drill a horizontal hole in an optional depth of a pit by fixing a scaffold smaller than the diameter of the pit to a substantially horizontally extending support member, a locking member and the rib of a plate, and supporting a drilling machine thereon through a turn table.

A scaffold 4 is formed into a substantially circular integrated structure having a beam 6 as the center. A support member 5 is inserted from the inside of the beam in such a manner as to be slidable, and the tip is fixed to a plate 9. The support member 5 is then extended and contracted to put the plate 9 on a rib 3 in the periphery, and a bolt is passed to fasten it. A drilling machine body 14 is supported on the scaffold 4 through a turn table 12. The body 14 is driven in this state to drill the bedrock around a liner plate 2. A horizontal hole can be drilled in a different depth within a pit 1, and the drilling of horizontal holes can be collectively performed after the excavation of the pit 1 and the arrangement of the liner plate 2.