To realize drilling by an automatic machine by surely detecting breakage of a gun drill having a breakage detection signal wire to be used in drilling a hole in a windshield glass of an aircraft.

A gun drill comprises a steel tube 21 and a handle 22, and is joined with a silver brazing part 24, and bores a hole in a metal, etc., by being rotated. A copper plate 21 is provided on an end part of the handle 22, one end of a conductive enamel wire 1 is soldered, the enamel wire 1 is embedded in a surface of a cutting blade 20, the steel tube 21 and the handle 22 in an insulated manner, while the other end is connected in the middle of the cutting blade 20 with a conductive adhesive material. A DC power source 3 and a sequencer 4 provided in an automatic machine are connected between the copper plate 2 and the gun drill, and when the gun drill is broken in boring a hole, the enamel wire 1 is also disconnected at the same time, energization is interrupted, the sequencer 4 detects it, and the operation of the automatic machine 60 is stopped, and breakage of the gun drill is surely detected to eliminate defective products.