A strain detector wherein the efficiency of a detecting coil is improved to improve the sensitivity of the strain detector. The strain detector comprises a magnetic layer made of a soft magnetic material having a high permeability and fixedly mounted on an outer periphery of a driven shaft strain of which is to be detected, and a detecting coil disposed around the magnetic layer with a gap left therebetween for detecting a variation of the permeability arising from strain of the magnetic layer caused by an external force applied to the driven shaft. The strain detector further comprises a metal layer made of a non-magnetic material having a high electric conductivity and disposed in the gap between the magnetic layer and the detecting coil. Or else, the detecting coil has a pair of coil portions connected to each other by way of an intermediate portion thereof, and an electric current flows in the same direction through the coil portions.
