To provide a rotary tool detecting method that is capable of properly detecting trouble with this rotary tool accurately at real time in a simple structure, and the device.

A threshhold value (high) of load in a normal condition of a drill and a threshhold value (low) of load in an abnormal condition both are stored in an operational means which incorporates a load value signal Xn from a load detecting means (S1), calculating a variation ΔXn in the load value and its standard deviation σΔxn (S2). In the case of an expression of Xn<ΔX bar-±3σΔx, it is so judged as a normal machining state, the threshhold value (high) is selected (S4), and it is compared with the load value Xn (S5). In this case of another expression of ΔXn>ΔX bar ±3σΔx, it is judged that the abnormal condition has occurred, and the threshhold value (low) is selected (S6), and it is compared with the load value Xn (S7). In the case where Xn exceeds each of these threshhold values (high and low), an anomaly detecting signal is outputted (S8).