A sensor with a strain gage 106 is disposed on an inner circumferential surface of a distortion generating body 100 which is a half-blindly hollowed cylinder having a prescribed thickness and diameter between a driving flange 101 to be fixed to a rotating body and a driven flange 102. The sensor converts a physical quantity corresponding to torque acting on the cylinder into an electrical signal. And the electrical signal is further converted into an optical signal by a light emitting element 113 and transmitted to a stationary section. The optical signal transmitted from the rotary section is made incident on a circumference of an optical fiber 115. The light made incident on the circumference of the optical fiber 115 is transmitted to an end of the optical fiber. The light which has reached the end is received at a light-into-electricity conversion element 131 to be converted into an electrical signal. Thus, a measured value of torque is obtained. Accordingly, torque of a rotating body can be measured in a noncontact manner with low electric power.