A tool damage /
abnormality detecting device capable of easily appropriately setting an allowable range for a value of comparison between a present value and a near past average for indexes indicative of loads on a tool. A
machining cycle counter index i is adjusted to i=0,
machining is performed as the loads are detected in a given period, and indexes H(i) (area S of load values, absolute value (G−) of a maximum inclination between two points obtained during a decrease of loads, etc.) are acquired for n number of cycles. Thereafter, the indexes and values of comparison between the index value for a present cycle and the indexes for the n number of cycles are acquired. A first comparison value can be used for initial value setting for threshold values. If any of the comparison values is deviated from the allowable range, the threshold values are updated so that the allowable range extends, and an update history is saved. If damage /
abnormality is observed visually or in any other manner, a threshold value update discontinue command is inputted,
machining is stopped after completion of the cycle concerned, and update values for the latest threshold value are collected to be used as set threshold values.