To provide a method and a device for diagnosing failure of a torque sensor, capable of performing a reliable failure diagnosis only by adding a circuit for diagnosis without adding a new detection section to a sensor section for diagnosis or changing the size of a torque sensor body.

This torque sensor 1 comprises a torque detection shaft 2 on which a magnetostrictive part A2a and magnetostrictive part B2b are provided as a pair, an exciting coil 4, a detection coil 3, a torque detection circuit 10, and a detection circuit 20 for diagnosis. A torque value is detected from the change in an exciting signal of the exciting coil 4 due to the changes in magnetic permeability of the magnetostrictive parts A2a, B2b and is compared to that detected by the detection coil 3. The failure diagnosis is performed from the torque value from the exciting coil 4 and that from the detection coil 3.