PURPOSE: To detect a cut-off with high reliability by making a first coil which forms a closed loop via a shear member, an open loop when said shear member is cut off and detecting change in the inductance of a second coil which is provided spaced from, and opposite to the first coil.

CONSTITUTION: A V-groove 13 is formed in the axial direction of an input side coupling 5 and an output side coupling 11, to fit in a shear member S, and a coil L1 in a coil body 19 which is fixed to the coupling 5, forms a closed loop via a terminal 18, a lead wire 17, the output side end part of the shear member S, the coupling 5, and a lead wire 20. A coil body 21 is fixed to the inner wall on the input side of a bracket 1, housing a coil L2, which is connected to a detecting circuit via a lead wire 22. If an overloading occurs in the input side causing the shear member S to be cut off, the coil L1 is opened, an induced current due to the magnetic flux of the coil L2 is shut off, the inductance on the side of the coil L2 is changed, producing an output in a detecting circuit and enabling the cut-off to be detected.