PURPOSE: To automatically and accurately detect whether a condition of drilling performed by a drill of small diameter is good or bad by integrating the drive current of a motor for driving the drill over a period of the drilling, comparing the integration value with a threshold, and detecting a breakage of the drill.

CONSTITUTION: A current supplied to a motor 2 when drilling operation is performed is converted into a direct current by an AC/DC converter 3 and outputted through a filter 4 to an integration circuit 5 as an electric signal SF. On the other hand, when a position detection signal SP1 enters, a timing signal output circuit 11 outputs a timing signal ST to an integration circuit 5 until a position detection signal SP2 enters. The integration circuit 5 integrates the electric signal SF over the period during which the timing signal ST is input and as soon as the rear edge of a pulse portion P1 of the timing signal ST falls down, an integration value signal SI indicating the integration value at this point of time is outputted to a comparison circuit 6. When the voltage of the signal SI is beyond the scope determined by the threshold in the comparison circuit 6, a signal SD is outputted to an alarm circuit 8 to give an alarm.