A detecting circuit 10 of a magnetostrictive torque sensor for detecting torque imparted to a magnetostrictive material 2 which has been subjected to shot peening is provided with a detecting coil 11 provided around the magnetostrictive material 2, and a drive unit 12 for performing AC excitation of the detecting coil 11, and is configured to detect the torque imparted to the magnetostrictive material 2 on the basis of a change in the inductance of the detecting coil 11, wherein the drive unit 12 performs AC excitation using a frequency at which the skin effect thickness is at most equal to the effective depth of the shot peening.