PURPOSE: To improve the response in torque detection by displacing the illuminated position of a light receiving surface in conformity with the twisted amount of a shaft member, detecting the displaced amount, and detecting the torque of the shaft.
CONSTITUTION: The light from a light emitting element 15 which is fixed to a shaft member 13 is cast on a PSD which is also fixed to the member 13. The displaced amount of the illuminated position which is proportional to the torque of the shaft at the light receiving surface is detected with a detecting circuit 19. The torque of the shaft is computed with an operating circuit 26 based on the detected amount of displacement. In this way, the torque can be measured regardless of the stop and rotation of the member 13, the torque can be measured in real time and the response is improved.