PURPOSE: To eliminate risk of sensor malfunction due to secular change and chips from machining and sense the part machied easily by furnishing a cutting part sensor which counts electric signals indicating the position of a conductive layer generated by a transducer, and connecting this sensor electrically with the body of a diagnostic device.

CONSTITUTION: A machining condition monitoring device for a drill 7 is composed of a pressure contact panel 8 situated at the undersurface of an elevating/ sinking plate 3 and contacting a multi-layer printed circuit board 2 when the plate 3 sinks, a sensor 11 which is installed in an appropriate place on the plate 3 and which converts vibration of at least the printed board 2 due to cutting with the drill 7 into electric signals via the pressure contact panel 8 and elevating/sinking plate 3, and body 13 of a diagnostic device which is connected with this sensor 11 electrically, analyzes the electric signals given thereby, and diagnoses the drill condition if it is broken or chipped. A machining part sensor 14 to count generated electric signals indicating the position of a conductive layer on the circuit board 2 is electrically connected with this body 13 of diagnostic device through an analog/digital converter 15.