A torque sensor detects a torque acting on a torsion bar provided between a first shaft and a second shaft on the basis of a magnetic flux density led to a rotating magnetic circuit portion from a magnetism generating portion. The magnetism generating portion includes a back yoke and a ring magnet. The back yoke includes a fitting portion fitted onto the first shaft, a joint portion to which the ring magnet is joined, and a stress reduction cutout that suppresses a transmission of a deformation stress generated in a radial direction of the fitting portion to the joint portion.