PURPOSE: To provide a sensor for a drill which can detect accurately torque applied to the drill with small sized and simple constitution.

CONSTITUTION: An exciting coil 12 is disposed concentrically on the outside of a drill 1 to span both sides of an edge part 2 of a drill 1 and a shunk (a shaft portion) 10 not formed with a groove. Further, a first detecting coil 13 is provided in the edge part 2 forming position side outside the exciting coil and a second detecting coil 14 is provided on the shunk 10 side. When AC voltage is applied to the exciting coil 12, predetermined AC signals are outputted by both detecting coils. Then, both detecting coils are differentially combined with a synchronous rectifier to be zeroed under no-load condition. When torque is applied to the drill under such a condition, the edge part side is mainly twisted to change the magnetic permeability by inverse-magnetic strain effect. Thus, the combination factor is changed to change the output on the first detecting coil side, so that the output of the synchronous rectifier becomes a value corresponding to the torque.