PURPOSE: To excavate a reaming while holding a hole wall by providing a plurality of step reamers capable of projecting along the peripheral surface of a casing and operating the reamers to ream after excavating the hole by using the casing.

CONSTITUTION: A case auger 1 houses a screw auger 11 in an external casing 8, and the external casing 8 has a plurality of step reamers 2 along the surface thereof. The reamers 2 are provided so that they are capable of projecting by opening windows by driving reaming blades 14 supported with the opening windows mounted to the casing by hydraulic cylinders 13. After boring a hole in the ground by using the case auger 1, projecting the reamers 2, and reaming steps 4 having a plurality of steps are provided into a bored hole. Cement is filled into the excavated hole to be reamed. According to the constitution, a reaming can be made while preventing the breakdown of a hole wall.