PURPOSE:To prevent a top sheave from projecting out of an arm end when not in use while enable the fine position adjustment thereof when in use by providing a normal/reverse rotation steplessly fixing device on a leaderless foundation forming device. CONSTITUTION:In a leaderless foundation forming device, an articulated arm A mounted on a rotary machine base 2 on a crawler 1 is controlled by a microcomputer to vertically lift up/down an auger B on an arm end 7a to dig a vertical hole. In this device, a driving shaft 11 perpendicularly intersecting to the arm erecting/collapsing plane of the articulated arm A is pivotally installed on a bearing portion 10 provided on the body of the arm end 7a and the base portion of the supporting arm 15 of a top sheave 16 is fixed to the driving shaft 11. By operating a normal/reverse rotation steplessly fixing device C provided on the driving shaft 11, the top sheave 16 is revolved up and down on the periphery of the side of an auger machine 20. Thereby, the position of the top sheave 16 can be finely adjusted, while also enabling the top sheave 16 to be housed when not in use.