PURPOSE: To prevent the return of excavated soil by a method in which a circular expanding blade is pivotally attached to the periphery of the screw blade, and the circular expanding blade is housed when the auger is inversely turned by a stopper and a coupler but automatically expanded when the auger is turned normally.

CONSTITUTION: An expanding blade 6 of a height smaller than the pitch of the screw blade 2 of a screw auger for on-site piling work is pivotally attached to the periphery of the screw blade 2 in such a way that it can expanded when the auger is turned normally but shrunk when the auger is turned inversely. The cross section of the blade 6 is a circular. A stopper 8 is provided to near the periphery of the blade 2, and a hook 12 fixed to a rod 13 vertically moving in parallel with the auger shaft 1 is coupled to the tip of the blade 6. When the auger is turned inversely, the blade 6 is shrunk by the stopper 8 and the hook 12, but when the auger is turned normally, the blade is expanded by means of the resistance of soil by releasing the hook 12. The return of excavated soil in the expanded diameter portion can thus be prevented.