PURPOSE: To enable increasing of the titled diameter easily even by means of a small driving device, by a method wherein, after a hole is excavated by means of an earth drill, the bucket of the earth drill is removed, a diameter enlarging excavator is attached, the lower it, and the bottom of a pile hole is excavated for an increase in diameter.

CONSTITUTION: A stand pipe 9 is installed in an open hole, and excavation is effected by a drilling bucket 4 attached to the tip end of a Kelly-bar 3. The drilling bucket 4 is pulled up to a ground surface to remove it from the Kelly- bar 3, a size enlarging excavator, set to the forward end of the Kelly-bar 3, is lowered to a hole bottom. A driving force is produced by a rotary driving device 5a, and a bit 5b is unfolded to perform excavation for enlargement of the diameter. This enables the Kelly-bar 3 to produce a reaction force when enlarging excavation takes place, resulting in the possibility to enlarge diameter without applying an excess load on a member and even by a small driving device without difficulty.