PURPOSE: To reduce facility cost and processing cost by carrying out deep hole and mouth processing in one process by arranging a bush plate in front of a deep hole processing position and installing a mouth processing cutter at a process depth position of a deep hole processing drill.

CONSTITUTION: A bush plate 11 is lowered before the start of processing, and the central axial line of a bush 15 to be installed is aligned with the center of a deep hole. Then, after a drill 8 is advanced and inserted in the bush 15, the drill 8 is rotated and advanced, and at the time when the bush 15 is processed to a process hole depth, the drill 8 is once retreated. Then, when the drill is retreated and comes to be in the state where it is extracted from the bush 15, a chip removal work in the hole is carried out, and the bush plate 11 is moved upward to the position where the bush plate 11 does not disturb movement of the drill 8. Thereafter, the drill 8 is advanced again and hole processing is carried out to a specified depth, and immediately before completion of processing, a mouth cutter 19 makes contact with the front surface of a processed article W and mouth processing of the deep hole is carried out.