PURPOSE: To enable muddy drain and surplus soils to be made as less as possible and enable the desired position of a hole to be enlarged, by inserting a forced board into the hole with advanced excavation, and by pressing the forced board against the hole wall side to enlarge the diameter of the hole.

CONSTITUTION: The hole 5 of the smallest diameter for inserting a forced board 4 is previously excavated by an excavator with an excavation edge 2 fitted at the tip of a rod 1, and mixing drum 3 fitted at specified upper and lower sections. Then, after the forced board 4 fitted on the rod 1 via a slidable rod 7 is inserted into the hole 5, the forced board 4 is pressed against the hole wall side to enlarge the hole 5. Then, the slidable rod 7 is removed, and the rod 1 is pulled down by a specified distance to enlarge the hole, whereby a pile hole 9 is obtained in the vertically same diameters. By this method, the excavation diameter of the hole can be made as less as possible, and the bearing force and quality of the pile can be improved.