To form a cylindrical hole in a ground by opening and closing two drilling cutters in the shape of slantly cutting a cylinder with a hydraulic cylinder for drilling.

When cutter ends are pushed into soil in the opened condition of a bucket and oil pressure in a hydraulic cylinder 4 is fed to the pull-in direction of a cylinder rod, the cutter ends 1, 2 are moved to the closed direction of the bucket to trap soil. After trapping soil, the bucket is lifted up, soil is discharged and drilling is finished. By repeating such procedures, a hole is drilled at a preset depth. The cutters 1, 2 are rotated on the axis of a center line and the contact of the lower end of the cutter 1 with the lower end of the cutter 2 shows the closed condition of the bucket, when the upper end face is nearly elliptical internally contacting the upper end face of the bucket in the opened condition. The hole is completly cylindrical above the upper end face of the cutters and nearly elliptical thereunder.