PURPOSE: To prevent lowering of depth accuracy in counterboring by separating a counterboring tool from a counterbored surface each time when work of one counterboring pattern is finished, and thereby cleaning chips therefrom.

CONSTITUTION: A mechanism 25 for controlling dust collecting operation is interposed between a chip collecting port 3 and a dust collecting device 24, and during the process of one pattern, counterboring is processed in a state that dust collecting operation is weakened or suspended by means of the mechanism 25. After one pattern of counterboring is finished, chips are collected with dust collection function intensified or actuated in a state that a counterboring tool is separated from a base board 15, and after the chips are collected, the following counterboring is started.