To reduce the life loss of a rotary blade device by accurately determining the time of breakage and replacing the rotary blade device immediately before breakage by detecting a clear suggestive wave generated immediately before breakage of the rotary blade device in cutting.

When a work W is cut by a drill T, load current driving a spindle motor M is measured by a high frequency current sensor D and is made an actual load current waveform. The actual load current waveform is AD-converted, variance of the change quantity of the peak value and the bottom value is calculated for each turn of working, and when the peak exceeds a predetermined threshold value for three continuous times in the graph of the variance, the waveform having this continuous peak value is recognized as the suggested wave. Since this suggested wave has a clear waveform appearing immediately before breakage, working is stopped immediately and the rotary blade device is replaced.