PURPOSE: To suitably keep the edge tip load of an excavating member by disposing a variable relief valve in a pipe line for connecting a pipe line, which serves as the inlet side of a hydraulic motor when an elevating member is lowered, to an oil pressure tank, and setting the oil pressure in an operator cab.

CONSTITUTION: In a pipe driver adapted to elevate an elevating member suspending an excavating member fitted to a reamer by driving a hydraulic motor 14 in rotation, pipe lines 20a, 20b are provided for connecting the hydraulic motor 14 to an oil hydraulic pump 10 for supplying oil pressure through a directional switching valve 12. A variable relief valve 22 is disposed in a connecting path 21 for connecting the pipe line 20a which serves as the inlet side of the hydraulic motor 14 when the elevating member is lowered and the oil pressure tank to each other, and oil pressure is set by an operating member provided in an operator cab. When the descending speed of an auger screw is excessively high, the preset pressure of the variable relief valve 22 is reduced to decrease the descending speed and lower the edge tip load. After the edge tip load is set to a suitable value, the edge tip load is kept within a suitable range only by holding an operating lever on the descend side.