PURPOSE: To perforate at a peripheral edge of a cutting face in parallel to a peripheral wall eliminate the need to perforate with an insertion angle so as to excavate a specified cross section of the cutting face, minimize overbreak, reduce the amount of explosives and the amount of blasted rocks to be brought out and the amount of concrete which lines the peripheral wall and thereby to enhance work efficiency and reduce excavation cost as well.

CONSTITUTION: A down the hole drill D, whose outside diameter is adapted to be less than a bit diameter (d) and, which is provided with each of piping connection sockets 18, 19 and 20 or accessories, such as rotary motors 23 in the rear end, is installed to a carriage 10 loaded on a guide shell 3 movably back and forth in such a fashion that the accessories may be located on the side of the carriage 10, thereby eliminating such portions which are higher than the bit 16 on the guide shell 3.