PURPOSE: To enable an expansion bit to be properly expanded, regardless of excavated soil resistance, and further enable an excavation work to be undertaken to provide an expanded hole having a target diameter.

CONSTITUTION: This expansion bit device is constituted of a drilling rod 3 connected to the forward end of a rod body 2, an expansion bit 4 joined to the outer surface of the rod 3 in such a way as freely rotatable, and an operation rod 5 inserted through the body 2 and the rod 3 in such a way as freely movable vertically to convert the vertical motion thereof to rotary motion for transmission to the expansion bit 4 and for the rotation of the bit 4 around a horizontal shaft 30. Furthermore, the bit 4 is forcibly rotated to properly expand, regardless of excavated soil resistance. An excavation work is thereby undertaken to provide an expanded hole having a target diameter.