DRILLING DEVICE

PURPOSE:To make it easy to reduce the diameter of a wing pit so as to prevent clogging with sludge by projecting the end portion of the wing bit inwards to form projecting portions that overlap each other when the diameter of the bit is reduced, and tilting their opposite surfaces, and providing a reverse water hole. CONSTITUTION:Water and air, etc., are fed forward through water holes 6a, 17a and are fed to a forward bit from the water hole 32 of a head stem 12 and discharged forward from the water holes, and a part of them is discharged backward into a steel pipe from a reverse discharge opening 40 to discharge sludge in the steel pipe. When a drilling rod is retreated, the head stem 12 is moved relatively to a sleeve 13 and a wing bit 21 and a projecting surface 21b is moved out of contact with the outer surface of the head stem 12 and a guide protrusion 25 is withdrawn behind a projecting portion 23; therefore, the wing bit 21, pushed inward by the end portion of the steel pipe 10 or the like, sinks into a storage portion 19. The wing bit is quickly enclosed because the clearance angle (alpha) of its outer surface is as large as 25 to 40 degrees. When the diameter of the wing bit is reduced, the diameter of an entire drilling portion 7 becomes smaller than the bore of the steel pipe 10.