To prevent an error detection owing to a continuity by the interference of a cutting oil between a drill tip and an opposing detecting bar, when the presence or absence of a breakage of the drill bar is detected by the continuity and the noncontinuity of both members.

A detecting lever 31 is squeezed from the lower side of a drill 5, and when there is no breakage of the drill 5, the drill 5 and the detecting lever 31 are continued through a box form of cover electrode 36. When the drill 5 is broken, the cover electrode 36 and the detecting lever 31 are made in the noncontact condition and not continued. Even though a cutting oil interferes in a gap G between the drill 5 and the cover electrode 36 so as to continue both members, the cover electrode 36 and the detecting lever 31 are not continued, and there is no error detection to detect that the generation of the drill breakage owing to the interference of the cutting oil is to be no breakage.