A strain detector in which a pair of magnetic layer patterns (5, 6) made of a magnetically permeable soft magnetic material are attached to a driven shaft (1) and a pair of detection coils (8, 9) are disposed to surround the respective magnetic layers for detecting a change in magnetic permeability of the magnetic layers which is caused when an external force is applied to the driven shaft. A magnetic yoke (15 or 16) is disposed around each of the detection coils (8 or 9) for promoting the passage of the magnetic flux therethrough, thereby decreasing flux leakage. The magnetic yokes (15, 16) each comprises an axially extending tubular member (15a) and radially inward outer and inner flanges (15d, 15e) extending from axially outer and inner ends of the tubular member, respectively. The magnetic yokes each may be made of a compacted core material containing magnetic powder and binder or assembled from a plurality of parts. A highly electrically-conductive, non-magnetic material (23) may be disposed directly between the inner ends (21b ) of the tubular member (21a) of the magnetic yokes (21) and the detection coils (8, 9) for magnetically isolating them without the need for any inner flange while allowing a compact structure.