Abstract: An apparatus for monitoring a machining state of a drill according to invention includes: a pressure-contact disk adapted to come into with a board or plate to be machined when a vertical movement plate moves downwardly so that a sensor can be mounted or attached with ease and there is no erroneous operation of the sensor resulting from aged deterioration or chips; a sensor provided on the vertical movement plate and adapted to transform vibration of the board to be machined, produced by machining by the drill, to an electric signal through the pressure-contact disk and the vertical movement plate; and a diagnostic apparatus body electrically connected to the sensor and adapted to analyze an electric signal from the sensor to diagnose the state of fracture or breakage of the drill.