PURPOSE: To obtain a means for directing a strain wave form in cutting to a tool holder casing which holds a tool holder so that the tool holder can be freely advanced in the axial direction of a cutting tool, by fixing a strain detector to a tool holder rotatably holding a cutting tool.

CONSTITUTION: For example, a semiconductor strain gauge 17 constituting a strain detector is fixed to the outside periphery of a spindle 3 of a drill 1, and is connected by a wire 21 to a rotatable ring 19 rotated as one body with the spindle 3. A fixed ring 25 is fixed through a bracket 23 to a part of the tool holder casing 11 which part corresponds to the ring 9, both of the rings are made to constantly contact each other by energizing them by springs or the like, and a detection current from the strain gauge 17 is directed to an amplifier 29 by a wire 27. By this means, the strain wave form in cutting can be directed to the casing 11, and even an emergency stop of machining can be automatically performed.