PURPOSE: To provide an abnormal load detecting system for a tool such as a drill attached to a tool holder, which can at once detect an abnormality when damage or overload is detected for the tool during machining.

CONSTITUTION: A tool holding body 11 is supported to a holder 2 so as to be circumferentially and axially movable, and first and second springs for urging the tool holding body 11 forward in the axial direction, are provided in the holder 2. A displacement probe 25 and a reference probe 31 which are meshed together are provided respectively to the outer peripheries of the holder 2 and the holding body 11. There is provided a detecting means for determining whether the displacement probe 25 is circumferentially shifted from a load position to an overload position in accordance with a load condition of a tool 12, and whether the holder 1 set in a suitable position range during normal machining, and the displacement probe 25 for detecting a circumferential shift generated between the probes are out of a suitable positional range or not.