PURPOSE: To efficiently transfer the rotating force and striking force to a steel bar by increasing the breakage-resistant strength of a shank rod and a cylinder, and tightly holding the steel bar of a drilling device.

CONSTITUTION: A clamp device 1 holding a steel bar 6 inserted into a hollow drifter is constituted of a shank rod 2 having a through hole 21 inserted with the steel bar 6 in the axial direction and connectable to the hollow drifter on one side, holding teeth 3 arranged around the steel bar 6 on the free end side of the shank rod 2 and coupled with the shank rod 2 so that it can hold or release the steel bar 6, and a cylinder 4 coupled with the holding teeth 3 and axially movably fitted on the free end side of the shank rod 2. When the cylinder 4 is axially reciprocated, the holding teeth 3 are moved near to or apart from the steel bar 6 to hold or release the steel bar 6.