PURPOSE: To detect the time when the performance of a core drill is deteriorated due to the loaded chips thereon by providing a control means outputting a core drill performance degraded condition indicating signal when the pressing force of core drill member is larger than the value set preliminarily during drilling operation of a platelike material.

CONSTITUTION: A drilling condition detecting device 32 for core drills 52, 56 for a boring machine is provided with distortion gauges 61, 75 and a control part 82. The gauges 61, 75 detect the pressing force of the core drills 52, 56 being under drilling operation for a glass plate 50. The control part 82 compares the pressing force of the core drills 52, 56 detected by the gauges 61, 75 with the preliminarily set value and outputs the core drills, drilling quality deterioration indication signal when the former is larger than the latter. Accurate detection is made in this way so that the drilling quality management is made possible.