To prepare a bulb surely by conducting expansion excavation by an expansion blade by forward rotating an excavation rod.

A plurality of expanding blades 7 are supported at the lower end section of the excavation rod in a freely rocking open-close manner so as to be rocked and expanded by earth pressure at the time of the forward rotation of the excavation rod and so as to be rocked and closed by earth pressure at the time of reverse rotation while an expansion control member 12 to the expanding blades 7 and a control lever turning the expansion control member 12 are installed respectively. The expansion control member 12 has closing inhibiting sections 17 inhibiting the expanding blades 7 at closing places for alternately working to the expanding blades 7 by revolution and expansion permitting sections permitting jocking to the places of expansion to the expanding blades 7, and the control lever is supported in a rockable manner so as to be reciprocated and rocked by earth pressure at the time of the forward and reverse rotation of the excavation rod while the expansion control member 12 is turned so that the closing inhibiting sections 17 and the expansion permitting sections are worked alternately to the expanding blades 7 by rocking at the time of the reverse rotation of the excavation rod.